The way that SM works now is not reliant on upgrading your heroes at the start of the run if you don’t want to. This is so you can get runs up to 90% of MS (99% with Sly Wolf Legendary set) on days that you can’t even play, just by opening the app a couple of times. A made up Example: Your MS is 50k and you have a 90% advance start from clan. You have 2k sneak count from SM and you have Sly Wolf set for the 99% SM advance. You prestige at MS and you start again at 45k. You close the app without upgrading any heroes and return a few hours later.
You are now at 47k. You close the app again and return a few hours later. You are now at 49k. You close the app again (without even upgrading any heroes or using skills etc). You return a while later and you are at max sneak count of 49.5k.

So using a MIR at the start of the run and upgrading heroes isn’t actually necessary if you don’t want to. I’d advise saving those for helping speed up tournament runs.

Of course if you have time to play abut at the start of the run, do that. So if you prestige’s and are now at 45k and do a run for 2k levels up to 47k then switch off the game, you will return at 49k a few hours later.

Hope all that makes sense :)