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

Also, check this page below if you want to host and deploy this in your own hardware/VPS/Raspi.
- https://docs.btcpayserver.org/deployment/deployment
It requires a full node(Must be fully synced)

Another source that includes video guides:
- How to deploy BTCPay Server on a VPS (Docker )
- Launch BTCPay Server via web-interface and deploy Full Bitcoin Node + LND
- Setting up BTCPay Server development environment on Linux (Ubuntu)
- Connecting your Wallet to BTCPay Server with xpub Ledger Nano S or other like Electrum
- BTCPay Deployment - One Click Setup via Azure

Other payment processors(Self-hosted and Hosted payment processors)
- https://github.com/alexk111/awesome-bitcoin-payment-processors

Have you used it before?