Dont we see that Marines are the ones driving the tanks and speeders and such?
Then you'd still need thousands more Astartes just to operate all their field equipment, you'd think.
Marines operate all the vehicles, but I think you're overestimating how many vehicles they have. From what I can find on a quick search, the ultramarines (who are almost certainly better off than the average chapter) have about 10 land raiders, 25 or so predators, 50 or so rhinos, and then a vaguely defined number of other ground vechiles like hunters, land speeders, and a small air wing. They almost certainly don't have more of the specialist vehicles than they do predators, however. Most of those vehicles have a a crew of 2, and I think the biggest ones like the raiders don't have more than 4 or 5, so you're only looking a a few hundred more marines, not thousands.
It's stuff like basic maintenance of mass equipment, the gruntwork aspects of operating the ships, and generally any task where being an Astartes is a negligible impact. Everything on the ground is Astartes when speaking of the Chapter proper, though some certainly have Chapter Serfs behaving as pocket regiments of Guardsmen.
That's not quite the case. Chapter serfs are trained to fight, but they're not typically employed as infantry by the marines, the only times they'll see action is when they're forced to defend the chapter's ships or bases from hostiles.
And basically even most support functions like logistics on field level should be handled by Astartes. Basically imagine deploying Astartes to a deathworld hellhole unaugmented humans can't survive in. So that means non-Astartes logistics couldn't follow up with the Astartes fighting force which leads to them having to pull out as they spend all ammunition, etc... So that's why relying on Chapter serfs is a bad thing. An army requires logistics to operate well, and infantry Astartes would need logistics Astartes to suppor them so they can follow them into battle providing support and basically keep up with them given the stamina and speed of Astartes.
Marines don't normally set up forward bases and the like on the surface, they handle resupply, repair, etc back on the ship. They also don't normally fight like that for extended periods, marines are a speartip, elite strike force, they try not to bog themselves down in conventional warfare because they don't have the numbers for it (if you ever have the time, you should listen to Arch Warhammer's seige of Vraks series, most of is about the IG, but marines do show up and you get a clear sense of how they prefer to operate).
Basically here is how I would organize an Astartes chapter:
The problem with that is those forces are actually way too big. Remember, for most engagements in 40k, a single company of a hundred marines + attached specialists is more than sufficient, and it's not uncommon for a company to split up into two smaller formations tactically. One of your battalions would be ludicrously overkill for nearly every battle.
It's also too big in the sense the imperium does not the marines to have this kind of power at their disposal, the secessionist forces as Badab had about that number of marines at their disposal, which turned out really badly when they went sorta traitor and the imperium had to go dig them out (and that was with several, not fully united secessionist chapters, not a single force with unified command). Remember, most of the limits the marines have placed upon them exist for
political reasons, not practical ones.