I had really only three key elements to my strategy:
1) Fish fast. I allowed one sim-week for fishing. I targeted specific fishing spots for specific things, but if I didn't get it pretty quick I moved on. At the end of the week, I went back and tried to get some of the fish/items I hadn't gotten yet, but I stuck strong to the one-week limit.
2) Take the space mission options most likely to lead to getting items, but once I had all the space rocks and aliens, plus the UFO fruit, I switched to taking whatever option was most likely to end the mission the fastest.
3) Used clubs to avoid taking time to eat or sleep.
This Site was really helpful:
https://levelskip.com/simulation/The-Sims-4-Walkthrough-Guide-to-Exploring-SpaceSome of the ways I ended missions quickly were:
Put the ball in the triangle hole - sends you straight home.
Decline the race. (I really always wanted to race, but saying no got my sim home faster)
Ignore the Populous Toilet Seat and just Go Home. (Doesn't always work - sometimes you're forced to stay)
Rescue the Pilot (instead of towing the ship) seemed to end quicker.
On the Derelict Vessel, where you can Board the Ship and then go to the Biology Lab or the Geology Lab, and you always get something - instead of boarding, you can Gather Parts and end quickly
Orbiting the moon ends quicker than trying to land on it
On The Green Man scenario, I more often "failed" (meaning the scenario ended sooner) when searching the swamp, so once I had the main items, that's what I chose.
There may be more, but I didn't write them down and that's what I remember.
These challenges that involve choosing between getting more of something or increasing the multiplier always intrigue me - trying to figure out which is the most important to do. A lot of the time I waffle back and forth. But with this one I made a plan and stuck to it - I think it was easier to stick to because it was a pretty straightforward challenge. But it did get really boring for me - three sim weeks of space mission after space mission with some "Rally the Troops', baths, and quick chats to keep needs up.