I’m a bit lazy and would prefer vanilla GNOME Web, though it’s been broken for so long that I’m considering that.
I installed it from Flathub using the PureOS Store. I can’t remember if I did anything special to get it to run but it does have a slow startup time. Just make sure you have a running version on Android or iOS as they require it to link the device unfortunately.
I don’t really like Axolotl in its current state. I think it lacks a lot of functionality and the UI needs work but if you’re just sending text messages, it’ll do the job. It didn’t seem to import my contacts but maybe I didn’t have my contacts stored in Signal so I had to add them with the phone number directly.
Flare looks a bit better with its UI but they have no plans on implementing calls, which is something I kinda need with my limited mobile plan.
There is a Nitrokey App from the PureOS repositories and from Flathub, though I don’t actually use my Librem Key for TOTP on my Librem 5. I would consider using it with my Nexdock with a larger screen and with a dock as there are only USB C ports without the dock. It definitely doesn’t fit on the Librem 5 screen so the Nitrokey App mostly unusable and as you can tell from the picture, the setup is janky without a USB C Librem Key available. The TOTP can be copied from the “Passwords” menu, though I’m not showing what’s under it for privacy reasons. 
I’ll have to give that a try. I haven’t tried Evolution either yet so maybe downscaling them would work.