It's unfortunate to report back that this likely isn't possible. Pausing the current Tick is not a nice experience on anything except for the Map Editor mode, since you're also paused and can't do anything. The Map Editor has special game logic that allows it to move and edit the world, but no other modes have that capability. In other words, all I can do in this mod is something similar to Time Tools (so you can install that to see how it would work), or swap you into the Map Editor (so you can run /editor
or install a mod for that). It's much harder to curate that experience, so I can't see myself going either of those routes.
The way I've always role-played this for myself is that your Sandbox is a deep bunker for scientists with a large do-not-disturb sign so as to not interrupt your thought process. You're cut-off from your reality on purpose, and you shouldn't be away for too long lest terrible things happen out there. I know that's not ideal for everyone, but for now it's the best I've got (absent other input or ideas).