I don't have any experience running Bazzite on a GPD device, but I know with my original Legion Go that everything except the microphone works and is supported to an OEM-level. As for if the hardware is worth it, it all comes down to the games she's wanting to play. If you get a refurbished 2023 model with the 7840U, that's the exact same silicon as the Z1 Extreme that's in the legion go, but it has double the RAM at 32GB. Handy because system memory is shared with the GPU and the 16GB of the legion is the biggest thing holding it back. That chip will play Baldur's Gate 3 at 1080p30 on medium settings just fine on Bazzite. If she's not playing anything too taxing then the newer models with the 8840U are gonna be overkill for her.
That said, if she doesn't need the hardware keyboard for anything, then the original Legion Go is a great device, works fantastic on Bazzite and can be had for pretty cheap these days.
I found these two pages by the way. They seem to say that the 7840U/8840U Win 4s have a suspend/resume bug but that everything else is solid:
https://docs.bazzite.gg/Handheld_and_HTPC_edition/Handheld_Wiki/GPD_Handhelds/
https://github.com/aarron-lee/gpd-win-tricks
I used the aarron-lee git page on the Legion to make my purchasing decision so his info is really reliable and up-to-date from what I know.
Edit: I should have added, the Legion Go S with SteamOS looks alright, but I would strongly recommend the Z1 Extreme edition at a minimum. That said, I wouldn't go for it because the trackpad looks pathetically small and I use it heaps. The Steam Deck's chip is also just horribly outdated at this point compared to its competition. My rec is the original Legion Go w/ Bazzite.