In your opinion, how big the business is going to be?
If you want to keep things simple, just use BitPay, I don't think much programming knowledge is required in that case. They'll take 1% of each transaction though so If you want to avoid that and save every penny, It would be better to implement BTCPayServer while running your own full node, you'll have full control.
this is not going to be a big business in the beginning but things can grow and I need a solution that can easily scale.
Actually I find BTCPayServer very interesting but then you really have to implement everything by yourself including the exchange part to convert in fiat and transfer to the merchant's bank account? How would you do that?