SSD sounds nice though. So here is a little pickle I'm in, along with my air, I also have an old windows desktop which I tend to use whenever something very windowsy work comes in. I have keyboard and mouse connected to a cheap hub along with a Time Machine HDD, and I would also attach the data storage SSD to this hub. Is there an easy way to share that hub between windows PC and Mac?
If that old Windows PC doesn't have Thunderbolt, and you get a Thunderbolt dock/hub, check whether the latter can work as a USB-C dock; it's my understanding my CalDigit TS3+ cannot, so if I wanted to use it with my old 2017 12" MacBook with one USB-C port...nope.
I'm not one of MacRumor's more 'guru-class' members by far, so take this with a grain of salt. I don't know of any dock being simultaneously used by 2 computers, whether Mac/Mac, Mac/PC or PC/PC. Some monitors have KVM functionality so they can switch between computers/devices; I don't know what happens when an external USB-C SSD is connected to a port on the display for use by the computer when you switch, though.
Digging online, I can't find where your anticipated display has KVM capability, so I'm guessing it doesn't. If it does, what you'd do is connect devices you want to share, like a wireless receiver for mouse and keyboard, to the display, and connect both your Mac and PC to the display using separate ports, then use the KVM to switch between them - that way the same mouse and keyboard would work either either device, without having to move a wireless receiver.
I've not researched external KVM devices; looks like they can get pricy.
With external SSDs (or hard disk drives, for that matter), be aware with the Mac you have some options for disk formatting, and a number of those are not cross-compatible with a PC. I think Fat32 and exFat are cross-compatible, but not what most Mac users favor, far as I know.
If you
really want to 'go down the rabbit hole' and have your computers share access to an external volume with files you work on, there's the NAS (network attached storage) option, but that's something I've only read and heard about, not owned and used. It would remain accessible to your MacBook Air when not docked, and you could enable online access to access your files remotely when away from home. Get one with more than one disk bay and you can have RAID provide backup. NAS is another whole area of computing. The brand that gets held out often is Synology. A NAS isn't just a 'cloud access' external storage drive; it's basically a 'headless' (no display) independent computer with one or more disks on which you can install app.s to enable varied functionality, like drive storage, backups, media server, etc...
Also, going from 5K to 4K, how does it feel to you? Any noticeable differences?
Got into that here -
https://forums.macrumors.com/threads/initial-thoughts-on-dell-ultrasharp-u2723qe.2443107/
Hasn't bothered me personally, but people vary. If you want a 5K 27", you're likely looking at a roughly $800 Asus or $850 Viewsonic.