I use the panels from perform and multi panel all the time, never had a problem with them, I prefer the panels in the shower/bath areas as it moves the job on quicker and they are easier to clean
If that was me doing the job, I’d remove the cast tee, and replace everything above in plastic, sealing the flange where the cast meets the new plastic tee
That’s not looking like a good install from the pictures
Fantastic boilers and back up service when problems occur.
Look at it like this how much do SpaceX & NASA spend on rockets and how many problems do they have, just because something costs more doesn’t mean it’s going to be fault free.