From
German Disney+, a scene that wasn't in the final episode that couldn't have been in the one we got since there was no time for The Doctor to go to a club before regenerating:
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Confirmation that Belinda's parents would have had a larger role about the actor playing her father:
Confirmation of the final episode having reshoots done despite having been finished far earlier in the year:
After Poppy and Belinda get out of the box they were put in the other characters have different outfits and hairstyles.
Susan never showed up despite her actress being old enough that her being able to appear in the 3rd season is in question.
All that by itself would normally be circumstantial, but given Gatwa's comments about leaving it seems like he just decided not to keep going after his contracted 2 seasons and so the final episode was reshot to allow The Doctor to regenerate.
The original ending likely would have had a cameo by Susan, Belinda's parents being involved onscreen, and some other answer for Poppy.
Belinda had already been established as someone who valued her independence and wanted to get home because The Doctor kept violating her autonomy and getting her into dangerous situations. Rewatch The Robot Revolution, the retcon scenes in Reality War about her being a single mother don't match up to what we saw there.
Belinda always being a single mother has no actual backing, it was made up wholecloth for the reshot The Reality War.