I'll take a shot at some perspective on this. Worth what you paid for it, just one man's way of trying to make sense of tech environment.
In a nutshell, I'd recommend not worrying about any of the issues you mentioned, at least not until your machine is closer to ten years old instead of brand new.
Exception: 32-bit applications. This subject is addressed at the end of this post.
Every year, Apple announces an upgrade to the OS for Macs, including its name. A few years ago, the new name included the designation macOS instead of OSX, but that did not reflect anything fundamental about the software.
Even though the software gets a new name every year, the new versions are incremental bumps from the previous edition. NOT like, say, Windows 7 to Windows 8, and then Windows 10 and Windows 11. Those upgrades turn the Windows world and people living in it onto their heads. Microsoft spent a few years trying the rekindle the spirit of revolutionary change marking the roll out of computers when the whole microcomputer situation was born. That is how they ran their train off a cliff with Windows 8. The Mac software has been through no such confusion. The upgrades are significant, but not gigantic bombs going off. Apple will run dramatic glossy ads, and youtubers will scramble to make new videos about whatever are the latest tweaks to the OS, such as dark themes, as if they are a huge paradigm shift, but to me, the changes are practically unnoticeable. To me, macs today work the same way they always have.
The main importance of the annual upgrades relate to hardware compatibility. Year by year, the changes work away from whatever mac you own, morphing into the hardware of the newer gear coming out. Since you are running a very recent model Mac (mac mini 2018), you can look forward to many years of compatibility with the latest OS from Apple. If you keep running that machine, in around ten years you will start finding aspects of the machine that becoming obsolete and finally, the day will come when the new OS will not include your machine on its list of supported devices. This a bigger deal for people running higher capacity machines, because the upgrades to high capacity machines from Apple costs a fortune. For instance, the new MacPros coming out start at $6000 for what might as well be an empty box. If you want any electronics in the box, you will need upgrades such that a reasonable configuration is running around $12,000. That is causing people to jump hoops in a manner somewhat like the subjects you mentioned.
OS upgrades can include significant technical elements. You mentioned one, the introduction of the dual-volume OS installation, where the read only system files are in one volume and other files are in another. Another big technical change in recent years is the new Apple File System (APFS). If you want to get into the weeds of those changes you can, but there is a reliable, easier way. If your model (including year) of Mac is on the list of supported hardware, all you need to do is wait for the public release, then run the update through the Mac interface, and you will be all set. The technical details are all handled by the installer.
You probably already know this, but here is a list of the new OSs released by Apple for Macs for the last ten years. While each came with a big new name, they all are point releases of version 10 of the OS:
Catalina 10.15
Mojave 10.14
High Sierra 10.13
Sierra 10.12
El Capitan 10.11
Yosemite 10.10
Mavericks 10.9
Mountain Lion 10.8
Lion 10.7
Snow Leopard 10.6
That pattern, where each new OS is a version starting with 10. spells STABILITY
The rollout starts with "Developer Previews" or developer betas, followed by public betas. The official public releases happen in September-October.
An important distinction between MacOS and Windows is macOS is made only for hardware sold and branded by Apple. Plenty of people run hackentoshes (macOS on non-Apple equipment) but Apple never goes out of its way to facilitate that process.
What that means is the annual updates always include updates that relate directly to changes and trends in Apple hardware. Last year, an example was the requirement that a computer have Metal-enabled graphics card. None of famous and highly regarded cheese grater MacPros were shipped with Metal enabled graphics cards. Apple included later model cheese grater machines on their list of supported systems, witih the caveat that graphics cards had to be upgraded to Metal. Now, cheese graters are off the list. When your machine is on the bubble,that will matter to you. Other than that, just run it, accept the upgrades .
A big "trend" worth mentioning is Apple's ongoing effort unite as macOS and iOS (the OSs for mobile devices). I regularly encounter articles explaining how that is happening, but i never notice it, possibly because Apple discontinued innovation of their mobile devices toward the end of 2011.
This is not 100% of the story. If you drop your computer into the bath tub while it is on, drop it down the stairs every day or run a monitor it doesn't like, you will have things to deal with. If you are into the Apple ecosystem, someone else will have to walk you through the intricacies, because its designed to make you see the world and do things Apple's way instead of being adaptable what I view as normal.
As for the Mac Operating system as it applies to a 2018 Mac Mini, I would say this explanation is not completely wrong. Others will disagree. I am unable to stop my brain from having views of its own, not originating or vetted by the crowd.
My first mac was a macpro laptop that died from heatstroke. It was not an OS issue and Apple finally corrected the problem in an extremely outstanding way. When it died, I was forced back to Windows. I learned from that, Windows is basically equal to the Mac OS in terms of getting my work done. I prefer Macs over Windows for a lot of little reasons. I am not going to spend $12,000 for a new cheese grater. A mac mini doesn't have the memory to run the Linux VMs I need, and not going to get an iMacPro. Instead I will spend around $3500 for an AMD motherboard and processor, a freight car full of memory, a few NVMe SSDs, a box tricked out with LEDs and fans, 650W power supply, a graphics card to run a few 4K monitors, a new 4k monitor to sit next to the one I have, and the two 8T HDDs already on hand that will run a lot faster than my beloved cheese grater made in 2012. Apple isn't going to make it a snap to run MacOS on that gear.
Running the macmini is turnkey. Just accept the updates, and let Apple sweat the technicalities for a few years.
UPON FURTHER REVIEW:
I forgot to address the subject of 32-bit applications. Here's the deal. Starting with Catalina, 32-bit applications are like the City of Atlanta when General Sherman rode his horse away from that city. A thing of the past.
If you have any 32-bit apps that you need to run, you cannot run Catalina. You could, but you probably would not want to, have Catalina and Mojave both installed on your mac mini. I remember thinking it was awesome that unlike Windows, having two versions of the OS on a machine is easy peasy. The catch no one mentioned is they cannot both be run simultaneously. You can boot one or the other. To switch, you have to power off and reboot. It's cool that you can do that but not convenient if you have to do it regularly as part of your workflow.
So, if you need to keep running any 32-bit apps, then you can just keep running Mojave.
