With the plastic, you could try to add more with soldering iron for extra thickness. I did my 'welds' pretty deep to make sure it wont crack again, but it's kinda risky to have it not burn through.
Also, same deal with my rear bumper holders back in the days I should have taken more photos,
here are some of the sweetest rust parts, ahh:
https://i.imgur.com/YUBB2zy.jpg <--- Bad
https://i.imgur.com/KcHEtOM.jpg <--- Really bad
https://i.imgur.com/eCAe5NN.jpg <--- bumper
Posted with links so not robbing space from your post