Oh experienced ones on the board, what am I doing wrong here...

Ground electro-plate off between the panels and where the electrodes will clamp
That is a must for sure
, sprayed with weld-thru primer.
Go easy on that primer. It is a "barrier" for spotting. Some brands (3m) are OK, others not so
If you welder has marginal power: maybe maybe a thin coat, or none at all. After all you're really adding a coating of Zinc there. Welder needs to "break-thru" that layer to fuse metal.weld.

Using Harbor Freight 110v spot welder, squeeze it tight, watch it glow red while I count to '7', hold it tight for a second or two, then release.
Your tip diameter look OK. A 120V unit can't do larger diameters unless thin metal. It's Current Density that makes a good weld. Layers need to be Real tight together. Use clamps close to where weld will be (both sides),

After I remove my last vise grip it pops apart. All the other spot welds seemed OK but now anything welded by that Harbor Freight spot welder is suspect.
1 option is to double up any existing welds, spaced in between existing. Or possibly re-spot weld them. But that could lead to a burn-thru.

Should I have used the 220v spot welder?
The minimum for any body panel work IMHO. If you're using an extension cord with that welder, use a 12awg or larger, with the minimum distance to the outlet. Less voltage drop to welder, more power out. Other thought is Duty Cycle: those are low by design. As they heat up, power output decreases. Last Resort: bring it back & tell them it's a piece-of-junk, BUT you're willing to try the 240volt model.