Alright, a Fire 7 bought in 2021 is probably an HD 7 (2019). If this is the case, then I had a search around XDA and found you this thread. It is a guide to rooting the HD 7 (2019) and installing Lineage OS. If you don't want to actually root and replace, then Toolbox is much simpler but will still require a Windows environment to run.I have a Fire 7 bought in 2021. Can I root it from Linux without using a Virtualbox?
Toolbox does not "root" your device, so if you want root you'll be better off starting in the link I gave. Toolbox only issues non-root ADB commands to your tablet to accomplish what it does.