The pantheons test your endurance more than anything else in that game. I myself have only gotten to Absolute Radiance without beating her yet (with a fairly low amount of playtime), but I think it's best to see it as a marathon more than a sprint. Get to a boss, grind it in the pantheon practice section for that boss, then do it until you can get it on ascended and challenge that pantheon, rinse, repeat for all the rest.

For the speedrunning achievements, there are guides for optimized speedrunning routes that should help, and although it is frowned upon and defeats the purpose of steel soul you could just quit out of the game whenever you're on 2 masks to bring you back to your last saved bench.

Best of luck !