I just want it to see end of 2022 so I can put classic plates on it, if even only for a yearYou should hose the area down with something like this...
You can't stop the rust, but you can slow it down by 85%.
Consider taking it in for a proper undercoating, even if just once.
Keep your car going as long as possible.
My gas tank rusted through, so I replaced it.
It was still worth the time, effort and money.
My frame is reasonably solid.
My other side rusted through in the same way as your car.
I stuffed an oil soaked rag into the area.
It sealed the hole, slowed the rust, and stopped any exhaust from getting in.