Realized my contractor installed my new TimberTech deck without joist tape, drives me bonkers. I'm currently debating whether I take the decking up to install or just roll with it.

OP, look up "joist tape" for this. Super easy to apply and will definitely prolong the joists.

PVC is so soft, it would scratch and wear immediately.

And it will just sit there, waiting, until something comes through, unlike oils and other products that evaporate or soak away.

What's your water flow rate? Do you ever take baths? How many gallons does your toilet flush? What fixtures are being drained into OPs line? What's their water flow rate? Etc, etc. Yes, as you say, code is designed to make sure it works regardless of the situation, but nowhere in this thread is there enough information to determine whether something less than code is ok. Not to mention the challenges ever selling if a prospective buyer figures out this is an unpermitted bathroom that doesn't meet code.

OP is not just talking about going to 2", they are talking about going to a SHARED 2" line which seems particularly bad.

A 2" line is less than half the capacity of a 3" line. Add in a sink or shower or something else draining when you flush and you are going to have a bad time.