Under the terms of the transition period the UK has to completely comply with EU rules, that would also imply that the continent also cannot change the rules to suit themselves.
Nope. That is specifically not true
The EU will continue to adapt its rules as it sees fit as it has always done, the UK has just thrown away its own vote and veto about that, but the UK only has the choice to abide by the changed rules or to quit the transition phase: That's it!
Yes, the EU will refrain from taking any unreasonable steps against the UK in this dependent state, but that was never in doubt anyway and would have gone without saying if there weren't those insane conspiracy theories in the UK. It's not a hard rule, though, but just restating the obvious.
It’s why you sent a revised bill for membership fees.
Wrong. That is just perfectly normal standard procedure and regular members are getting that, too, when the actual cash flow deviates from budget planning. You're just noticing that for the first time, apparently, which is saying something.
So nothing actually changes until either a trade agreement is reached or we go to the fall back position of WTO and third country entry requirements.
Or until the EU changes its rules normally as it proceeds. You're mistaken if you believe that the UK's transition phase would somehow freeze any progress of the EU. It does not
I’m still surprised that the EU allowed the ROI to enter into an agreement with the UK for CTA
Your surprise is still primarily based on not understanding how any of this actually works, despite all the available information, including as given in these threads!