Options for Heating A Small Room

I'm in the process of turning one of our 3rd floor rooms into an office/mancave, but I'm struggling to find the ideal means of heating the room.

I recently bought an electric Mica convection heater for the room, but it took about 5 hours to bring the temperature up from 50 degrees to about 63. That seems like a really long time, and I'm hoping to have the temperature closer to 70, and I'll probably only use the room 8-9 hours during the week. I want something that can bring the heat up relatively quickly; perhaps within 1 hour?

I would prefer an electric source and have been doing some research. It seems the ideal type of heat (if my goal is to heat the whole room; not just the desk area) is radiant heat. I've looked at a few electric oil radiators, but every single one seems to have a few negative reviews saying they've sprung a hot oil leak, and I'm really trying to avoid that.

Are leak-proof electric radiators a thing? haha

If not, what are some other options??

The room is roughly 135 sq ft.