Solus and Manjaro are shipping Snap installed by default and I've never had a problem installing snapd on fedora. All I ever had to do for that was run a single standard dnf install. Apparmor doesn't pose the problem you think it does
Snap is portable to other distros, look at the official website and you see a list of distros, you can use snap on. That doesn't mean that there is no vendor lock-in, just a different kind. Snap as a format grew out of Cannonicals effort in the mobile field. Snaps where supposed to be the truly convergent successor to click, the packaging format used by Ubuntu Touch. And this history is baked into its DNA. It's right there on the snapcraft website: "The app store for Linux". As such Cannonical has always courted proprietary software and/or software by big companies (VS Code was first released as a snap for a reason). I think that they have always have had an eye on one day adding app payments and the sweet, sweet 30% cut they can take from every sale
I think the primary fear seems to be giving Meta access to our user data. The concerns seem to be not completely founded but even then there is still the uneasy feeling with helping facebook of all companies with their next project
Your best bet is probably to get a refurbished thinkpad. I've seen X1 Carbons (their ultra thin Macbook-like line of ultrabooks) for 250 bucks on backmarket. That's the 2016 model (G5) and not in the best tier of refurbishment (So there might be some small scratches on the chassis) but the hardware should be more than enough for your usecase. Linux generally speaking does very well on low end hardware and thinkpads are especially well supported.
One thing though: DON'T INSTALL KALI ON YOUR DEVICE, IT'S NOT DESIGNED FOR THAT. Or rather it wasn't. Originally Kali was meant to be run of a thumb drive only. By now enough people have attempted to install it that the devs have given up and provide a bare metal edition. That is just Debian with some stuff preinstalled though. There is very little secret source (like the "undercover mode"). You should go with debian and try to build your toolbox as you go. This way you will get a better insight into your Linux system because it forces you to set up everything yourself
Solus and Manjaro are shipping Snap installed by default and I've never had a problem installing snapd on fedora. All I ever had to do for that was run a single standard dnf install. Apparmor doesn't pose the problem you think it does