I don't know how DNS seed works but for me, there is an easy way to implement a payment processor on your website that doesn't need to download the whole blockchain just to have a full node.

I suggest you check their documentation from here https://docs.btcpayserver.org/getting-started/
It is easy to implement than developing your own payment processor that requires a full node. BTCpayserver only needs your wallet "xpubkey" or Master public key on Electrum. You can also use your own full node/Electrum personal server if you want but if you don't want to download the whole blockchain you can use any server from Electrum.

Opensource: https://github.com/btcpayserver/btcpayserver