Always found it weird regular Starfleet personnel such as Picard & some senior crew and Sisko & senior crew were sent on top secret infiltration missions. You'd think they'd have a corps (perhaps not an entire division) of highly trained specialists to do just those kind of missions.
I think an issue would be is that... space is big.
They may have all sorts of special forces... but... they might not be anywhere near where they need to be deployed RIGHT NOW. If you can get them, great. But sometimes... you might not have the weeks/months it would take to get these people into action.
Especially in the TNG era, I go with the small, spread out Starfleet idea.
I do think the idea that Starfleet MUST have Marines to be a bit wrong though. Starfleets primary mission is not combat, but... Marines primary mission would be combat. They are somewhat antithetical to the primary mission. But EVEN IF Starfleet did see a need/want for Marines, there might be other implications. There may be legalities/politics in the Federation. I could see a scenario where Starfleet isn't really supposed to have ground combat troops.
The Federation has ground combat troops, but they me a bit more like a NATO situation where they're coming from the member worlds home forces, not the federalized Starfleet.
That being said, I tend to include Starfleet Marines, although as a somewhat small force and even then, alot of what they do is less direct combat and acting as more as "observers" who are specially trained in combat and would take over / train the repurposed Starfleet crews when needed.
One thing I do like to do is riff on something that IS canon but not really explained. We know the Enterprise-D is assigned to the "Deep Space Exploratory Corps". I've expanded on that myself and have Starfleet divided out among several fleet corps. There's the Deep Space Exploratory Corps, which is kind of like, the holy grail for most Starfleet officers. There's some like "General Operations Corps" or some such that is like the more Coast Guard/Police/Defense fleet, and there's a "Utility Support Corps" which is the Lower Decks "utility ships". They're all still Starfleet, just in different divisions.
First would be command, and involve all the various fleet operations and components of those fleets, and starbases.
Second would be training division, and be all the components of the academies, and campuses as well as training requirements for entrance.
Third would be Engineering and ship systems, including computers. Commonly called operations.
Fourth would be sciences and research division.
Fifth would be intelligence and counterintelligence operations.
Sixth is security and defense.
Seventh would be logistics and personnel.
By and large, I think that's how Starfleet IS set up, although I think it's just complicated by the fact that they aren't completely separated divisions and just all work together directly. So like, there's not really a science fleet and a security fleet and this and that. On ships/bases, personnel from all of these divisions work directly alongside each other.