Depending on your platform, this may be helpful:
https://github.com/CodeSolz/WooCommerce-AltCoin-Payment-Gateway. I have seen some sites using that plugin.
Honestly, I haven't seen many cryptocurrencies payment processors/gateway being developed, at least not as fully open source. BTCpay is pretty much renowned, but as you have seen, the development mainly focused on Bitcoin. I can only suggest BTCpay if you want to develop the payment support yourselves since the last time I see the altcoin addition/plugin lack of developments.