Pulsar is a fork of Atom under active development. We don’t publish a flatpak (yet) but there is a community maintained flatpak for it.
Otherwise if you want to look at something else I’d give Lite XL, Lapce or even Zed (it has now been open sourced and looks like it has a flatpak available) a look as interesting alternatives.
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.