I have made many projects with casters. The casters I use seem to get bigger
over the years, must be a universal truth in there somewhere.

The last thing I did wasn't amenable to bolting, but yet I didn't want to weld
galvanized. I spent a whole year fitting galvanized angle iron in a Navy ship
once, and I don't ever want to smell that yellow smoke again. I made up a strong
hot lye solution and dumped in the casters (after removing the wheels and axles)
and not only did it remove all the zinc plating, the free zinc also reduced the
rust to zero, so they went in funky and rusty and came out looking shiny and
new. I welded them on and then painted the bottom of the project with red
primer, and I'm sure they'll last for many years.

I'm not clear on what your concerns are. Drilling a bunch of holes is a pain.
Prepping for welding is a pain. Grinding off old welds is a pain if you ever
have to replace the casters. Getting rusted on nuts off of bolts is a pain if
you have to replace the casters. Done properly, bolts will be strong, done
improperly, they will be weak. Same for welding.

Do whatever you think is best.

GWE