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.
Yeah I'd like to run custom ROMs too but do devices like these even have custom ROM support? I'm OK with patching the image manually with Magisk every now and then but doing that for every security update is a PITA. If only there could be a way to automate the patching, backup and flashing process
Check out XDA forums or if you want an easier time, check out the lineage OS devices page. If its on the lineage site there's a good chance other OS's are available as well.
Is the problem just with mobiles then? I have never had an S series mobile but I've heard about how Samsung devices can't be unlocked even if they are not bought from a carrier
No idea. I avoid their phones but their tablets are the best option for solid performance and AMOLED so I buy them periodically on fleaBay. I suspect it's just a phone thing, I've unlocked every tablet I've owned.
Hmm, I suppose so, since carriers don't sell close to as many tablets. I too really like Samsung tablets but never considered them because I thought they'd be locked. Thanks
Just be aware that there's not much to flash on most models. There's not a large enough community to support an active ROM scene so you're often limited to just unlocking the device and patching the factory image with magisk, occasionally there'll be a non device-specific GSI treble ROM or something but that's about it.
System updates are a pain with a patched device too, it's all by hand work every firmware release. I usually use Frija to grab and decrypt the image and patch it with magisk before updating with Odin.
You use Frija to download and decrypt the full image then patch it with magisk on the device, then copy that back to a PC to flash to the device with Odin. It's :effort: but it's the best workflow I've found. You can download images from the various sites that host decrypted Samsung factory ROMs but Frija is significantly faster.