I don't know how hot a chimney gets, but would be very hesitant to saddle one with EMT, NEC 310 contains ambient temperature derating factors. Just simply seems like a bad idea, wiring adjacent to a chimney. I personally would try and figure out an underground route, even if it meant saw cutting a strip of concrete, or tunneling under a sidewalk. Run 1.5" PVC the whole way, maybe even 2" to make the wire pull easier. I have a 100A 220V sub panel in my garage and it's sure been nice, mine was easy, my main panel is 3' away on the other side of a block wall. It's really nice to have 220 when you want. I put in a 4.5 kW 220V unit heater I got free... plus I got the sub panel free too. Spent many years in the trade, mostly as a foreman (first dibs on demo haha). When I moved in here, I had the standard 15A single duplex outlet, fed from the master bathroom. Trip the GFCI, had to go upstairs to reset it. Much nicer now. I must have 6 or 8 dedicated 20A circuits, I have never once tripped a breaker or GFCI out in the shop. When I installed 2 new air conditioners, it was super easy to feed them from the sub panel, too. Invaluable.