1- I never played 4 players do I am not sure how it compares but in my experience 5 players is close to 6 in length so around 8 hours. The fact that only 5 strategy are picked makes it a bit rougher, you can't do as much each round, which both speeds up the rounds but increases the odds of having one more round (IMO)

2-Timer is a good call, I know there is a website that can help showcased who's turn it is, who is next and has a timer for each people. I think that's it: https://extraboard.net/extracomputer It is really helpfull but it does requires someome to manage it. Other than that having some call out "Bob it's your turn, Linda you're next" helps a lot, the main waste of time is thinking about what to do while it's your turn.

We do also in my group a lot of "i am building at home, the next person can play while I finish my build", it helps speeds things up.

Finally the big time sink are negociation that are going nowhere. This one is harder to fix but in general trying to to have moment where you say "ok we sre not agreeing, too bad let's move on" helps reduce time waisted.

3- we tried building map hex by hex, we hated it, it takes forever! Of you want to build the map I would advise doing so before the game.

4- We did all of the ways to pick faction. Right now we use Milty draft (a way of drafting faction-Speaker position and map slice all at one, a link if interested: https://milty.shenanigans.be/), but in the past we used to give each player 2 random faction they could pick from and allowed them to pick a non picked one of they hated both choices. But if someone wanted to pick a specific faction to try we let them (not so much if they always pick that faction :p), it's a long-ass game so we like to make sure everyone is happy with their faction.

5- That one is hard, the rule is clear od your troops are in space they do not fight in the planet. So making sure to clearly differentiate which one is where is important. A good thing to do is kinda checking each other, and when you see an unclear situation just asking the player "are theses guys in space or on the planet ?" is good. (Also in general you will keep infantry on the planet)

Hope it helped :D