I went Kindle way only because there is no 300 ppi 10-inch Kobo yet
I gambled, but was lucky to get one with FW <= 5.16.2.1.1.
If you plan to buy a Kindle that you want to get JaliBreak on, make sure:
1) Kindle is brand new and unopened.
2) Look at the box's back, find 16-character serial number. If you divide it into four groups of four characters, like AAAABBBBCCCCDDDD, pay attention to CCCC. These numbers are YWWD, where Y is the last digit of the year (for new Kindles, 1 is 2021, 2 is 2022 and so on), WW is the week number of that year, D is the day in that week. This is the manufacturing date of the Kindle. You need YWWD to be 321X or less, like 317X, or even 2WWD. This will ensure that the FW is <=5.16.2.1.1 for Scribe or <=5.16.0.1 for other Kindles - where getting JB is possible.
3) Because of the above conditions, probably your best bet is an open market like eBay where you can see the actual box before shipping (except mass sellers, unless they are kind enough to take photos for you before you purchase). You can also try your luck on Amazon, and then return unopened box if the manufacturing date is too new (or even opened, I remember they used to accept it as is anyway for full refund, don't know how it is now).
Then, with JB and FW, there other considerations.
1) You can get JB, then you need to block OTA updates, then install KOReader and other stuff. If it gets updated to a newer version, you can downgrade back and restore JB. Here you need to keep track of when this happens an never do two in a row of either FW upgrade or factory reset without restoring JB in between (otherwise it will be lost).
2) There are some positive moves towards making stuff useful on FW versions 5.16.3 and above:
https://github.com/koreader/koreader/pull/11986 . The thing is, Amazon switched compilation code libraries from soft float ones to hard float ones. This means the compiling tools and software compilation needs to be tweaked specifically for this change. I am about to test KOReader on 5.16.9 soon.