as someone who uses Affinity apps on both macOS and Windows, what sort of system will provide the most enjoyable experience comes down to how much money you want to spend.
Macs are always going to be expensive, off-the-shelf Windows systems with any sort of performance will be less expensive, but usually the specs are throttled in some fashion (great CPU/memory, low-end GPU, for example).
Build to order is still your best bet, if you find a company where you can configure a system online, and they'll build it for you.
My current Windows box was done this way, I simply couldn't be bothered building it myself, after too many years of doing so.
I'll echo the comments about RAM, more is always better. As far as GPU is concerned I have an nVidia RTX3070 which doesn't struggle very much (although my Affinity usage will probably differ from yours).
In my opinion, the best thing to do, is decide on how much you want to spend, and then look at what's available.