That's a massive space. Can you wall off a separate room and use propane? If not, I heated a similar large space using a wood furnace with excellent results. Of course, wood heat has it's own issues.

Harder warmup?

Harder warmup?

Frozen mats will be the issue. I'm also a Judoka, so hitting cold mats is crummy. Otherwise, extra warmup is good.

Our gym is probably twice the size of the space you're describing, and we used a smaller, cheaper, propane-powered version of this: https://www.lowes.com/pl/[...]1235867110

We used it for a couple winters, and now we have a larger, more appropriate unit.

When on top, stay on top.

At least for the first couple of rounds.

First off, how is your insulation, walls & ceiling? Good insulation will make a huge difference. If at all possible, frame up some walls and create a smaller space. It is far better and easier to heat.

If you are worried about cold mats, build a floating floor. I makes it where your mats are not in direct contact with the floor and easier to keep warm.