No chance of overwriting your bootloader when you distrohop using LiveUSBs and LiveCDs.
But srsly, Trisquel GNU/Linux is a distro that provides Ubuntu with all the non-free and contrib removed.
Parabola GNU/Linux-libre is Arch with all the nonfree packages removed.
Worth a look if you support software freedom and transparency.
No chance of overwriting your bootloader when you distrohop using LiveUSBs and LiveCDs.
Or learn a whole new system and try nixos
WARNING: you probably should try it out in a virtual machine first.