Feels not entirely underpowered compared to ARM alternatives, does it have a somewhat decent battery life though?

Looks like about 3 hours: https://forums.puri.sm/t/my-librem-11-arrived-today/21480/16

if only it supported sim cards

Then Librem 5 smartphone might be a better choice.

not really the same - that's sort of the nice thing about ipads and some business laptops is you can add a sim card and use them anywhere. The MNT Pocket Reform has this but the waitlist is months out.

Yes, but I already have an iPad and they have an excellent hardware and keyboard. I'm mad at the thought that I'd buy additional hardware instead of Tim Shareholder Value enabling a single boolean flag.

You made the wrong bet. Apple will never give its users freedom to run what they want. Free software can guarantee it though.