I have run into similar problems. Old PC, did a Win 11 update earlier in the year using Rufus to overcome the blocks, but reverted to Win 10 after a few days.
However I have had constant issues with Windows Update failing to grab or successfully install files to the point where it's choking the PC. Something fairly major borked as opening Task manager only lists the immediate apps running not all those underneath. So having run sfc /scannow, now doing DISM with a mounted Win 10 Iso file (first DISM attempt fell over) which has just finished!
Ideally i want to get Win 11 on there again but an attempt earlier today hung on the first reboot screen. Hoping the repair might fix that. Ultimately I need a new PC as this one is an ancient potato by any measure, but still putting together an investment case for the missus!