I hope your taxes system has a concept of only applying to certain item groups and/or certain specific items. Some places have food taxed, others do not, water isn't taxed, alcohol is taxed differently, food with enough sugar is taxed differently than just "food", and the list goes on.

Also your clients might want to be able to add their own "taxes" (aka service charges) for certain days/weeks of the year or by the channel that the order was placed.

POS system are incredibly complicated and even more-so if they span multiple restaurant industries or types. Think table service vs drive thru vs carryout vs curbside and things like pizza vs coffee. A lot of what you build will not transfer over. Your logic to handle half and half pizza probably isn't going to be reusable in other industries.

I will say one thing, it's never dull.