That is a nice business idea and you're gonna earn much; not just from trxn fees(pos charges) but also by indulging in fx so you could trade back your funds. Kudos!
But I guess you're gonna have alot of trouble cus a client is not gonna wait for a confirmation from the memphool,or maybe they will? It all depends.
I am not going to keep my customers waiting, once one network confirmation is done, I can allow the customer leave.
i was also actually thinking about introducing the LN to his customers. which is cheaper and maybe faster. but you need to orient your staffs or people that will directly be involved in this. for me, one or 2 confirmations is enough. some exchanges are crediting even just one confirmation. but giving the money without confirmation at all is very risky. try to orient your staffs with at least blockchair or blockchain, to see how the btc transaction is doing.