The game doesn't go on sale very often because it's very fairly priced at $35 and I've gotten each DLC the day of release so idk how they do bundles. It may be a long wait if you're hoping for a deep sale as I've never seen it drop more than 25% below retail price.

You get a lot of the core experience from the base game+mods. So don't feel like you need to get DLC to start because there's a learning curve on the game anyway. The community endearingly calls your first 1000 hours in the game "the tutorial."

The only must haves are QoL stuff like repricing up weapons, run and gun, jobs menu reworks etc.

Everything else is what flavour you want, Neolithic colony, wh40k gene editing , etc

Everyone plays a bit different, personally I'd recommend playing the vanilla game for a bit and using mods to flesh things out or iron out any rough edges in how you experience the game. The modding scene for the game is absolutely phenomenal, if at any point you're going "gee, there's got to be a better way to do X, deal with Y, or add more Z", there's probably a mod that does it, for example I like designing my colony fairly early on (so I have something to build towards), but since the existing mono-color plan gets confusing pretty fast (what was wall and what was workbench, tool cabinet, light, etc), I find More Planning to be a bit of a must have. As a blanket statement, the Vanilla Expanded mods are very well done and integrate neatly into the game (that said, they aren't necessary meant to all be run at once, so you can pick and choose what you want and go from there).

I have a pretty decent list going, but if we're just talking a short list of personal favorites, I really love Megafauna, Frozen Snow Fox's Bionics, and Cyber Fauna. (Oh and a shameless self-plug for my own mods)

Enh, I'm not so sure about that, one of the most unique parts of RimWorld is that the primary goal is to tell a story. Even the best stories need a bit breathing room for the action-y bits to have weigh. RimWorld is filled with stories about colonies that ran out of food in the dead of winter, lone survivor types that either bleed out or later died of infection after a freak hunting mishap, or trying to hide from the flames and wait out the raiders/ murder machines. It may be waiting, but I find that more often than not (especially in the early game), it's either a welcome break after a hectic day or an edge-of-your-seat fight-for-survival kind of waiting.