Stock typically comes with more preinstalled apps, does not scale, the comparison is also lacking since power users quickly can disable those background apps, that people often love to call bloatware. Again knowledge is everything as you can quickly tweaks aosp pretty easily and also get very solid OS.
You can also isolate apps with third-party apps that you even can get from F-Droid, island etc. there plenty… Once again you show knwoledge is key, not the OS.
There is old stuff there which is one of the reasons grapheneos does not directly deliver their apps trough the store, there are many other reasons, you quickly find the reddit thread to that.
I do not need to defend nor shit-talk f-droid, someone else did that for me already, just check my community I linked the f-droid article myself + wrote my own article about f-droid insecurity.