I had no problem unlocking my secondhand Tab S6. There usually aren't 3rd party ROMs or recovery images available though, software modification is limited to grabbing the factory Samsung ROM and installing magisk on it.

Not as bad as you'd think, especially if you buy from a place that tests the devices or know the person selling isn't hard on their devices.

If you are going for a model thats a couple of years old, I would recommend going an alternative OS and no google apps. The analytics they run run does make a considerable difference to battery life. You could use something like microg if some banking apps don't like not having google (the ones I use don't have this issue YMMV)

I'd also recommend simplifying how you use you're device. Ie, don't have apps that run in the background like Facebook, try and keep to using those sites in the browser.

Don't stream stuff over the mobile network(applicable to some tablets), as its a large battery drain. It's designed to blast a load of data and then go inactive to save battery.

You could start doing some of these things on your current device and see if you get much of a change in battery life.

The best device is the one you already have.

I unlocked my US variant WiFi A8.

I've unlocked my US Tab S, Tab S2, Tab S5e and Tab S6. I've never had trouble unlocking one.