"I am considering putting a thick steel plate on the bottom to lower the C of G and some better casters."
Those would be my first choices also - and LARGER too - casters that small tend to STOP when they hit a BB, not to mention anything larger - and the combo of sudden stops AND top-heavy is a perfect recipe for disaster...
These
4" Caster, Double Locking, Swiveling with 4 Hole Mounting Plate
Are my favorite REAL casters, got 'em on several tools - they come in 3-4-5" wheel, 4" is usually the smallest I'll use (BB effect) - they lock in BOTH axes. Not cheap, I typically "stock" a couple sets, Woodcraft has 'em on sale a couple times a year.
IF I were doing your cabinet (and did NOT need "offroad" capability), I'd build an angle frame, go "outboard" with HF's 10" solid rubber rear wheels (so I could do a "dropped axle" to keep things low (same with front casters) - the angle frame would leave room for a plate (lower CG) and if the angle frame interfered with bottom drawer operation, just cut that side down a bit - This one's for a Bridgeport, but it's what I meant by "dropped axle"
Doing it that way would keep tank swaps no more difficult than they are now, keep the CG as low as possible, make it easier to roll, AND those casters will make you wish you'd tossed the OEM's a LONG time ago
And this is my bandsaw, rear wheels are 13" no-flats - in that particular case I WANTED more height, so no dropped axle :thumbsup: Also, the rear setback is to keep the saw from "doing a wheelie" when it's in vertical position...

To keep your cart low, the "drop axle" would be a "Z" bracket outside the angle frame with a 5/8" hole thru the vertical part - then a 5/8 bolt (axle) thru the larger tire, and 2 nuts locking the bolt to the Z bracket (kind of the reverse of my bandsaw version)
The bigger better wheels would lessen the "bb" effect, the frame and plate would help lower the CG, and if you use the casters I linked you wouldn't have to scratch your head to figure out which way is "locked"; it's MUCH more obvious
If my description wasn't clear enough, holler and I'll do a sketch... Steve