I’ve just moved to Thunderbird. I was never keen on the old design and found it rather clunky but the new UI I find much better.
I was using Mailspring but it has recently just refused to work on my device and I never even got a response on the community forums so I’ve just given up on it.
I have an older, second hand Thinkpad Yoga but it has served me really well with Fedora installed on it.
I also have a PineTab 2 but that has been a little bit of a rough start and still doesn’t feel that great as a day to day device just yet
Honesyly I think I prefer the laptop form factor over tablet + smart cover. Even my old Android Asus Transformer I used almost exclusively in laptop mode to do anything productive.