Mr Hobby matte is the only varnish I’ve been really happy with for a truly matte finish.

I'll say, investing in a airbrush has been great.
Even if just for base coating and varnish.
The main reason is you can do it inside without stinking the whole place, which means not dealing with all the shitty weather conditions involved in spraying minis outside.
Too humid, too cold, too hot, bugs, whatever.
Nope, just sit down, spray, clean, done.
The propellant in rattlecan also melts a few types of foam (XPS?) and I use it sometimes for terrain.
Airbrush don't melt anything.

With the price of citadel cans you can proudly buy a airbrush for the price of 2-3 cans that'll last you forever.

I use AK Interatives matte varnish spray can and they also work fine but that too is a miniature hobby specific product. Of the hardware store products I tried I found a few which were okay but they tend to have bigger particles (the matte effect is cause by tiny particles to form a microscopicly rough surface) which can at times clog up parts of the minis. But they work alright otherwise.

A good option I found is from art supply stores. There are varnishes for sealing acrylic paintings. Those work very well from my experience as there was no large particle issue. Prices are among the same or higher as your typical hobby products. Probably because it really is the same thing. Or so I suspect. So if you have an art supply nearby they might be able to hook you up when the LGS is all out. As for brands, I'm in the EU so I probably can't be of help.

I like Testors Dullcote but it’s hard to get here; I try to buy it whenever I see it. Dulux have one called Duramax that I like. Have tried several other hardware store clear sprays and haven’t been happy with them.

Testors dullcote would be my recommendation. Should be able to source through your local hobby shop, or online through somewhere like Amazon.