Little update after a average few weeks with Covid / getting annoyed with the car.
Did a temp fix on the rust - after hitting it with a wire wheel, it went through the metal in a few more places, revealing 3-4 pea sized holes, through to the firewall near the fusebox.
Will definitely get properly fixed in the future, but for now, I wire-wheeled it back to bare metal then patched / treated using filler and POR-15. Bit of an amateur-hour repair, but it should delay the rot a bit and ensure my carpets don't get damp - will check underneath again in spring.
Found the source of my front suspension clunk too - went away when I removed the swaybar (suspicions were correct) - likely hitting front subframe or transmission guard due to a little bushing play / bar angle being off since it is reasonably low on standard end links.
Going to get new poly bushings and billet reinforced brackets and when I reinstall will make sure suspension is loaded and clearances are fine.
Interior reupholstering still in progress, hopefully will get it back next week - door cards are getting fresh vinyl as well with the insert stitched in. Ideally would have kept stock vinyl on door cards, but will be the same as the outer bolsters and seat backs, so should tie in nicely.