Your use case is not bad at all. I suggest to analyze first each merchant case and adapt the specific solution proposed that suits better.

LNbits, BTCpay or any other BTC/LN wallet could serve well, but not always are the solutions for everything. Sometimes even with a simple mobile LN wallet a merchant can just start accepting, without any other server / node to setup. But then come the problem of liquidity (see the guide about private LN banks).