1 to 5 Stars- Accessibility: There are readily available materials to learn from on the Internet for players; ie videos, pdfs and the like.
1 to 5 Stars- Material: Does the setting have well established lore that aids with my use and the players’ use of it? Is it dense or are there a lot of “fill in the blanks.”
Yes or No: Pregen modules and characters for ease of entry.

Lancer
&
Battletech.

Do they like DnD 4e or 2e?

5e. We tried 5.5e but they weren’t vibing with it.

Lancer is 4e with big stompy robots

Battletech will make 2e with all the optional rules look simple.

Everyone gets puppies vs diarrhea forever ahh gm (I did not enjoy battletech but I am really enjoying Lancer.)

There is also star finder, which is pathfinder in space (3.5 +) and i think stars without number, which… I don’t actually remember anything about other than it being a simplified system in my memory

I know it's not exactly the question, but Cyberpunk was a ttrpg long before it was a video game. Cyberpunk Red might be worth looking at.

Paranoia is a scifi dystopia and might be fun for a short campaign.

I was going to say starfinder. My group researched this and was going to switch to it before we fizzled out. No experience, but looks great.

I've done a couple scifi one-shots using GURPS and found it super easy to run

Important note for BattleTech:

BattleTech is a tactical wargame. Primarily built for 1v1 scenarios where you each control a handful of mechs. Very little RP built into the system.

What you'd be looking for is the MechWarrior TTRPGs which are set in the same universe. Either "A Time of War" or "Destiny."

Destiny is gonna be a very narrative, rules light system. Much more focused on telling a story. Quick and easy, and very collaborative. That's gonna feel different for the players and GM if you're mostly familiar with DnD.

A Time of War is gonna be more crunchy tactical wargaming. Lots of dice. Like DnD it's mostly built around combat, but it's gonna be more in-depth, closer to the DnD 3.5 levels of crunch.

Lander FWIW is very fun and pretty easy to get into. There are some good prewritten adventures that do a good job to introduce both players and the GM to the system.

My group has tried Starfinder 2e (we primarily play DnD 5e) and we did not enjoy it. On paper it all looked fun and like things we'd enjoy, but in practice it's just too crunchy and obnoxious to keep track of everything and to look up individual rules for a bunch of different actions and conditions, even at the lower levels. It also didn't really evoke much of a sense of scifi for us, but that can probably be chopped down to a bunch of little reasons that might not be a fault of the system.

Lancer: Accessibility **** Dense Rulebook, but CompCon make building characters/ Mechs easy.

Setting. **** Dense, but open. That said, the system is built into the setting.

Pregen: Yes? Iirc, No Time For Wallflowers has pregens.

Battletech: Accessibility *** A Time for War is pretty unfriendly but overall, the rules are easy.

Setting **** Dense but open again. You can dive into the minuta of what worlds manufacture what components or the logistics of a mercenary company, but there is plenty of space and time to do your own thing- generally.

Pregen No? I'm not aware of any.

I really like the Alien RPG. It's a simple d6 system, the stress mechanic is cool, the written adventures are fun, and you don't have to commit to a single character in the written campaigns.

Why do they want to play mech games? Do they like the anime drama or do they want to play crunchy combat?