Installing it on an external drive at first is an excellent idea.. That is what I will do, although I'll wait until the .1 version is released. But because I require all my third party software to be compatible with an OS, I will (most likely) not install Big Sur on the internal SSDs for each of my Macs until either the .3 or .4 version comes out. That is typically the time when a version of Tech Tool Pro will be available (that is always the last of my third party apps to have a version ready).
None of that is an issue for me, as 1) I can still test Big Sur on my external SSD, 2) Catalina runs just fine, 3) there is nothing earth shattering in Big Sur that I must have, and 4) I can be on the sidelines reading about all the experiences folks are having with Big Sur, and thus be well prepared when I finally do "upgrade" (will do a clean, fresh installation of OS 11.3 or 11.4).