Don't worry about it. The FD is collectively hated and badmouthed only in the tech bubble on the internet, for example on forums like these. The real world effect between a FD and an SSD is noticeable for sure, but it's not as drastic as many make it out to be and for most users it is absolutely and 100% negligible.
Case in point: I've recently purchased a 27" iMac 5K i5 refurbished for my daily work. After I'd had it for a few weeks I actually installed an external SSD via USB-C and ran macOS off that SSD instead of the internal FD to figure out whether it was worth the cost and hassle. Yes, it was certainly faster but no, it was not worth the extra investment in a 512 GB external SSD (the one I had used was a 256GB SSD I borrowed from a ThinkPad). Truth be told once the iMac had been running for 1-2 hours I couldn't tell the difference between it being booted from FD and from SSD anymore. It was much slower booting up and launching applications for the first time from the FD but that evened out after a few hours - basically once I had all my applications running and in memory. After that there was absolutely no difference anymore. The most important thing is to tell your bookkeeper is to never actually power the iMac off but to put it to sleep instead. It'll be back up within 2-3 seconds and since applications are still in memory they will launch instantly.
I use mine for software development in Java, system operations & development, writing technical documentation, occasional bookkeeping, remote system administration, etc. It's not the heaviest of workloads but it's certainly more than most people do.
