rattrattus it's great you're making sure to run GOS in the future, welcome to the community!
The SIM cards you were able to run on the Pixel 4a should run without a problem on any other pixel running GOS (or any other phone available in your country). There's no functional limit caused by GOS or the newer Pixel phones that I'm aware of. If you buy a device not meant to be sold in your country (e.g. from China) they might not have the right antenna to support all the LTE bands in your country, but that won't be a concern for your situation and goals.
As mentioned above buying a new Pixel phone directly from Google will give you the best guarantee to have a GOS-ready device. You might even get a nice kickback if you trade in your Pixel 4a (make sure to factory reset it first). Please do not buy a device older than the Pixel 6 (see here).
I'd like to add a higher privacy suggestion if you like to not let Google know what device you buy and where you live. You buy a new, unopened non-carrier Pixel phone from a local store (preferably further away from your home location) and pay in cash. Keep your face covered (e.g. with medical mask plus hoodie) and away from video cameras. You then open and unlock OEM right there on a public WiFi, put it to Airplane mode and take it home for GOS installation. This comes with the following benefits:
- You have a very high probability of getting a working device (and if not, you can instantly return it).
- Google never gets any identifying data that it could connect to you until this point (not even your WiFi identifiers when unlocking OEM).
- There's no payment or postal trail leading to you in case this specific device gets tracked later on.
- If you wait a bit, you might get greater discounts on the device compared to discounts on Google's online store.
In order to keep your privacy and anonymity levels high, you'd need to reconsider some things (such as where you get your SIM card, what information you give and how you treat apps and Google Services on your GOS-phone afterwards). But this is the best start I could come up with if you want to be private and anonymous right away. It might also be total overkill for what you want to achieve, so please make sure you know your threat model and not listen to nerds like me when all you want is to quickly buy a secure device that can run GOS. :)
Edit: Added SIM card info.