Upgrade to 4.4 - Ajax error / 90.2% completed,

[kjadams]

I am having a terrible time upgrading from 3.10.12 to 4.4 as well and received the same Ajax error / 90.2% completed, and then a bricked site that displays no content and has no admin login.

I suspect I have an old template (3rd party, purchased from Crosstec), but it's not clear to me how to fix that.
I suspect my extensions also need to be updated, but it doesn't seem like Joomla knows where to find them. The Update tab on the Extensions page says "There are no updates available at the moment."
I have fixed the bricked site using Akeeba backup / drop databases a few times.
I have tried following this instruction set: viewtopic.php?f=812&t=988003 but it seems like there is something missing between step 5 and step 6 (the dev site in the Joomla4 folder works until I edit the configuration.php file)
I have used Akeeba magic eraser, I have admin tools installed. But it seems like I have templates and extensions that need to be updated but I am not finding any luck on how to do that.

Server is running php 7.3.33. I have cPanel and Database access. Any advice you could give would be awesome.