Unfortunately I have no experience with BTCPay server.
From a quick look at the docs, it does not seem that the API it exposes is the standard RPC API, instead it is a different API altogether: https://docs.btcpayserver.org/integrations/customintegration
So it seems unlikely that you will be able to connect with the available coin adapters. (Although I’m not 100% sure, I could be missing something here.)
Moreover, it would not be easy to develop a new coin adapter for it, because in this model, the client application (WordPress in this case) should be holding the private keys, not the BTCPay server. It might still be doable, I guess the coin adapter itself can hold the keys, but it’s a very different architecture from what we had before.
The good news is that since you already have the pi, if you have enough storage on it, you could simply install a (pruned) bitcoin core node on it, and this will definitely work with the built-in Bitcoin core coin adapter. Just remember to refresh the user deposit addresses after you connect to your new wallet.
Please let me know if you have any questions about this.
P.S. Were you unable to complete KYC verification? If you can get verified, then CoinPayments should still work. If for some reason you cannot get verified, then I guess it’s time to switch to a full node!