TeX on Arm Infrastructure
As it says on https://www.tug.org/mactex/aboutarm.html

After installing MacTeX-2020-Universal, all front ends and the command line programs will automatically use Arm binaries on Arm machines and Intel binaries on Intel machines.

so if you start from scratch and want a working TeX installation which runs natively just install the Universal package and you are all set. The remarks about the 2021 version providing full ARM support most probably refers to extended testing and maybe adapting some programs to make better use of ARM.
As for upgrading to 2021 next year, this will work the same way as before: The 2021 package will install into its own directory within /usr/local/texlive and you can then switch to the new version with the TeX Live Utility. Any user-installed styles and fonts are in /usr/local/texlive/texmf-local so they will remain available.
PS: Using Homebrew has the advantage of updating TeXLive automatically whenever you update your Homebrew installation. OTOH Homebrew currently is still transitioning to ARM so IMHO it's better to install MacTeX directly and run tlmgr update --self; tlmgr update --all every few weeks to keep it updated.