BTCPay does indeed look quite promising. Unfortunately though it's not designed for sending payments which is actually a crucial requirement for us.
I've also looked into Electrum (maybe in combination with the CLI), but I'm not sure if this is scalable enough.
You can send payments from BTCPay Wallet (
if you're using a Hardware Wallet) or the main wallet where the xpub was extracted from.
The hardest requirement to fulfill is the 1m+ address which is only possible with Bitcoin Core (
req. the whole blockchain).
Electrum will struggle to sync the wallet's transactions, don't know if their own BTCPay Wallet and Wasabi can.
If BTCPay Wallet can handle all those addresses: Purchase a Hardware wallet instead.
For Bitcoin Core: There's no official documentation to connect Bitcoin Core to BTCPay:
docs.btcpayserver.org/getting-started/connectwalletBut since it only requires the main wallet's master public key, it's possible to connect it to Core.