Not sure I really count as a cost conscious tech bro so much, but...
One of the things that I've noticed is that Android devices really don't get real-world OS updates past about the two year mark, or two major Android revisions, from the manufacturer. Correct me if I'm wrong, but that seems to go for both phones and tablets across the board. So, any long term support to keep the device going past the usual planned obsolescence deadline will automatically require LineageOS, which means shopping by device with existing mainstream LineageOS support.* This will both make sure you have a longer potential firmware update support window, and massively simplify device selection given how few tablets LineageOS officially supports.
* Unless you really want to go through all the pain and effort of doing the builds yourself every few weeks to manually update it instead of using a pre-compiled and signed binary, or deal with unofficial rando builds from randos on the internet over at XDA Forums who may or may not release further updates after a one-off build.There are security disadvantages to unlocking the bootloader for installing third party firmware, but the issues that implies can still be mitigated to an
extent, though never technically done away with, once you break that seal. This said, if you don't root the OS, you're picky about what you install, where you go, run the internet through
AdAway and/or use an alternative browser with adblock like
Fennec with uBlock, keep it at home behind a well configured firewall and use WiFi AP isolation, and avoid using the thing to do sensitive things like financial stuff, that may or may not help you sleep better at night... and it beats losing security updates.
Of the manufacturers of Android devices, Nokia and Motorola/Lenovo seem to put out some of the most robust hardware and vanilla Android I've messed with. I've personally not been that impressed these days with Samsung's build quality or their default junked up Android builds, but when you narrow down the field to LineageOS device support, that really only leaves two devices made within the past two years, both of which are Samsung Galaxy Tabs (
S5e and
S6 Lite), and three devices made in the past five years with the Lenovo
Yoga Tab 3 Plus. And no, I'm not eliminating other manufacturers, there's legitimately only three tablets (though two models have LTE modem variants, bringing the full total to five whole devices if you want to be pedantic) made within the past five years with official on-going LineageOS build support.
Certainly makes selection easier.
Although ARM64 mobile processors have been pretty beefy the past five years when combined with a lean AOSP type build, newer processors and GPUs can still perform better than old, and the internet is progressively becoming more GPU acceleration dependent. So, given you requirements, I'd lean more towards the Samsungs out of a beggars-can't-be-choosers necessity and not out of preference. Of the two, I'd personally choose the S5e as it's not custom Samsung silicon, and it's already closer to EOL on firmware updates and coming out of warranty given the 2019 release, which'll translate to cheaper used models and less guilt just kicking the official bloated Android build to the curb, as there's no warranty to lose after putting on your "I am the warranty" t-shirt to flash LineageOS. Extra bonus, the "non-user replaceable" battery probably won't be too worn with the newer stuff. Plus, you're giving an already built device a second life instead of buying new in the current market with all the caveats that implies.
If I didn't care as much about performance, I'd probably get the Lenovo if I could find one.
Them's my freely given two pennies, Paul. Do with it however you see fit.