I’ve bought a kindle Oasis about a month ago and have been reading LNs on it. Indeed reading manga is a bit annoying since they’re low res scans of the pages and you can’t select text you might not know, or zoom too much on kanji that you don’t recognize at a glance.
As for tips, first thing that I did was set it up to be in japanese, and then added a JP->EN dictionary. Tried to use to build in JP->JP dictionary, but I don’t think I’m there yet in terms of being able to understand a word in japanese using japanese without taking too long or having to look up example sentences. I think it’s the one Jisho is build on, so it’s quite good. You have to download it from your PC and put it on your kindle, not sure what the path was unfortunately… think it’s just ‘your kindle/dictionaries’
Another tip would be that, when selecting words I would try to do it as I would with smartphones, but you have to sort of keep pressing for a bit longer and drag the finger down to grab more text ( since something the text selector finds the first word it think you want to find, which doesn’t work too well for compound words ).
Also as other have mentioned you might need a vpn in case digital purchases are blocked the first time around. ProtonVPN has a JP server on their free VPN account type if I’m not mistaken.
I like checking this website ( https://jpdb.io/ ) every now and then for LN suggestions. Ultimately it’s best to read what you want to read, if it’s not too difficult to the point that it’s not fun to read. I’ve been reading through KonoSuba and ひげを剃る, I find both to be fairly fine for my level, which KonoSuba being a bit more difficult, but nothing too crazy.
Also take a look on what LNs are available through the Amazon Kindle Unlimited subscription. Maybe it’s better for you to pay for the subscription, rather than buy each volume individually. I think that if you read about 2 volumes per month it’s more worth it ( also I think there was a Golden Week sale going on ).
Finally ( unless I remember something else ), there’s a feature for highlighting sentences when reading. These get saved to your kindle, and you can review them whenever you want. You can also get them by connecting the kindle to the PC ( they’re in a text file called ‘My Clippings’ ). So if you want to make flashcards from then, or review the sentences some other time you can do it.
Hope this helps I really do like the kindle even if I do most of my reading on my PC.