You may want to check your building codes. Some places require a garage to have 5/8" drywall, usually mandated IIRC if it is attached to the house.

The old might be 5/8". The only place I can measure is in the attic
opening and it's hard to get to. It was either 1/2" or 5/8" but when
I measured I was just seeing if it was thicker then 3/8". We just re-
did the kitchen, tore out a dropped ceiling and it was 5/8" in the
kitchen what we removed.

In your case since it is the detested popcorn finish, just replace the whole ceiling to save on labor. Once the old ceiling is removed, the joists can be leveled for a good fit for new drywall. A rented lift can get it all in place in a couple of hours. Good luck.

I think the big thing is clearing enough floor space in the garage to
access everything. After 30 years the garage is a collection.

I agree with Joe...Remove the old rock,strap the ceiling 16OC shimming where necessary under the strapping and re-hang with 5/8 Fire Code drywall to bring it up to code. Your house insurance may even go down a little......Good luck...

Not sure what 16OC shimming is.... ???

Old brittle sheetrock that has been bowed for a long period of time will never go back up and stay for long....

I did put some screws in at a few spots a long time ago and had to put
washers on the heads as to pull the sheet rock back flush it would
pull the head of the screw thru the sheet rock. I guess we have some
work to do. Biggest issue is space. Garage is filled with old
kitchen cabinets we're going to use for garage cabinets but won't be
able to work on ceiling over them when installed. Kind of a catch-22,
to make space need to put cabinets in, but to put cabinets in need to
do ceiling, to do ceiling need cabinets moved to make
space... ::sigh:: Home ownership is an adventure! :O)

Thanks for everyone's help!