You might be able to run a really basic Linux on the older devices, but their very limited RAM will severely constrain what you can run on them.
The native OS is so optimised for the hardware, trying to get better performance, or even matching power consumption on generic Linux is just not realistic.
Frankly other than for hobbyist purposes the last version of the native OS is probably as functional as they’re likely to get.
Having said all that, more recent iPads are powerful machines that are likely to still be very capable fur as long as they will run. Asahi Linux is compatible with M-series hardware.
So while think getting latest gen performance such as modern browsing out of legacy kit is a pipe dream whatever Apple did, I think there are real possibilities going forward.
If the answer is "just don't have a tablet and buy a FairPhone or a feature phone, have less tech", I think that's coherent.
But if the answer is just "don't buy Apple, but continue to buy", I'd say they're all just as bad at best, way worse in general. Apple devices have a longer supported life, although others tend to be more hackable.