How to make center dash pieces fit better?

Haven’t seen major issues with those panels. Usually the double sided tape takes care of it as long as you clean the heck out of the tape spots with alcohol. You might compare them to the originals and make sure there is no warping.

The top tabs generally seem to hold ok, but the problems can come with the lower tabs and the double-stick tape. I've had mine on & off several times over the past couple years, and have noticed they occasionally do not want to seat flush (I prefer to NOT use double-stick tape), Next time I get ready to put them back on, I'm considering drilling a few holes along the bottom so I can use some push-pins to secure them instead of the double-stick tape. Even unpainted, I don't think the push-pins would be that noticeable since they're down pretty low. Just make sure any holes you want to make won't interfere with any wiring or other parts.
I guess color-matched screws would also work as long as you're careful to not overtighten and strip the holes.

Clean the plastic with soapy water then alcohol. Sometimes stacking two pieces of tape works better.

I bought this stuff to help the 3M tape stick better...

3M Tape Surface Primer 94 1/2 Pint for Vinyl Di-Noc, 8 Oz 21200242168 | eBay