The tomatoes I grew which lingered longest in small pots in the greenhouse were mealy too.

Sungold, for example, I planted out very late and the first few trusses were not very good, but latterly (they're still in production) much better.

A couple of pots of gardener's delight were left in tiny pots in the greenhouse and produced several trusses, but they weren't ideal either.

No problem with flavour, incidentally, just texture. All outdoor ones, planted in good time, were fresh and juicy.

Oh, if you have a problem with mealy tomatoes, don't, whatever you do, try "green sausage". They make lovely sauce, but are not for salads.