I have succesfully upgraded several RPi's to Bullseye following the guide below. But they all did boot from standard sd cards and I had full backup of those for each before I started. The upgrade worked well and all are still running fine. Only thing is if you have a lot of other experimental stuff installed, that will of course also still be there and sometimes it is nice to start with a fresh install. In my case I had software installed that would require me to rebuild them on each RPi, rebuild that takes very long time, so in my case, I saved a lot of time
I'm not sure if it still apply but not all adapters perform the same on a Pi
See this link
To this day it can still be treacherous to buy a storage adapter for your Raspberry Pi 4. There are many that will not work properly and perform very poorly.
Still showing £6.98 for me on Amazon-UK (although in the picture it says £12.28 - ignore that).
I ordered three SSDs plus the case as I needed the value of the order to be over £25 to get FREE delivery.
If you wanted MORE, note that CCL have a 1TB M.2 device going for just £50 right now. That's a steal and I ordered one yesterday for my Tiny PC workstation. That tiny thing will be (is already really) my most powerful computer ever despite its tiny size. 32GB RAM, i9, NVIDIA Quatro, sweet. Not a server though, it is my daily-driver - benefits of working from home.
As it seems you allready made a decision - I just can say - I had several buster to bullseye tries on raspberries. None of them succeeded - most of them done short after bullseye was available for the RPi.
I guess I did close to 100 updates on PCs and servers - they generally did succeed.
In my cases all new clean setup RPis did speed up extremly - at least it fellt that way.
What I can suggest (from more then 35 years of PC work)
Take a paper or notepad (or at least a medium that does not rely on one of the services you will have to turn of - Dont get me wrong - JUST to be sure
Write down every tool or service you use
copy the configs
copy the data
I do even write down the paths - mostly digital
copy all the settings on the machine - mostly the things you forget that they where touched once
grab all crons
grab all desktops if you used a GUI
If I do want to feel very secure I do this and grab the history to make a small sh script out of it
not very sophisticated - just with oneliners for each task
but this makes some kind of documentation and very often you then see what still missing
(and if you really need to - the setup gets better and faster each time to redo it )