Firstly, make sure the fit is OK. There are plenty of Youtube videos about how to get road bike fit, you certainly don’t need the full on £££ pro roadie geometry checkover.
Change things gradually, small increments and try to only change one thing at a time so you know the effect it has.
Imperfections in fit show up far more on road bikes cos you’re in the same position for long periods of time unlike on MTBs where you change position much more and generally stop more, faff around, ride a bit, drop saddle etc. As mentioned abovce, most MTBers start off by being way too rigid on the bike. Try to relax, drop your shoulders, change from hoods to drops regularly and keep smooth on the bike.