This all turned out a bit rambly, gonna send it anyway. I hope you can get some positives out of it if you do decide to read it. The last three (real) paragraphs are more structured, so if you’re only gonna read part of it, make it that part.
I have noticed that playing on Foundry (or roll20 or any other VTT software) often leads to more video-gamey/war-gamey campaigns. VTTs assist the DM and players with a lot of the gamey aspects, but offer much less assistance, if any, for the other aspects of ttrpgs. This leads to a lot of the potential issues you described. In my experience, having virtual battle-map after battle-map (or a single large one) thrown at you leads to reduced creativity, especially in newer players who do have experience playing computer games.
It takes both time and conscious effort to realize that what’s in front of you is not the actual game being played but just a projection. This counts for both sides of the DM screen. Some people will never clear that hurdle or even realize it’s there. That is fine for some people who do love the video-gamey aspects but not for others who want the opposite, or at least more of a balance.
For example, personally I also DM a Pathfinder campaign on Foundry and I constantly have to remind some of my players that they don’t need to click on all the relevant buttons the system already has programmed in. They can just say what they’re doing and do a naked roll if finding the button takes more than a second.
In regards to your last paragraph. You did get an in-game reason from your DM, although they were a bit indirect with it. Your DM telling you you might have to roll up a new character also means your in-game reason is: “I feel like I might die if I try this.” My advice in that specific situation would be to lead by example and roleplay your character going through those thoughts/emotions. I know it can be awkward, might even get a giggle or two from the other people around if nobody else has done any roleplaying yet this session or campaign, but someone has to make the first step.
Also don’t forget that the players have a lot of power in shaping the game. You do not have to explore a huge cave. You can walk out at any time, go to a town and talk to people instead. It’s kind of a dick move though if you let your DM prepare this cave for days, letting them believe that’s what you were interested in, only to then not engage with it. Of course that requires all the players (or at least a good number of them) to be on board. To find out what you all want to get out of this game together, communication outside the game is required.
I’m gonna be devil’s advocate for the last three paragraphs of my rambling response:
Do you actually KNOW that the enemies you’re fighting are arbitrary? Do you know there is no explanation for the mimic and the gelatinous cube? No explanation for the moths and larvae? No explanation for the Morlocks? Is it possible that your characters just haven’t found those explanations? I’ve placed enemies in my games before and the players never found out WHY they were there (the characters never even bothered thinking about it in some cases) but there was always a in-universe reason. And even if your DM didn’t initially have a reason, I would assume they will be able to come up with one on the spot.
For example, Mimics are described as: “A mimic can remain in its alternate form for an extremely long period of time, sometimes remaining disguised in a dungeon chamber for decades. Regardless of how long it waits, the mimic remains vigilant and alert, ready to strike at any moment.” They do seek to ambush adventurers so hiding in a cave that would be attractive to adventurers is the #1 thing they’d do. It could’ve wandered in 10, 20, 30 years ago, biding its time. Saying something like “I investigate the area around where the mimic was, trying to find any hints whether it has moved recently” after combat could lead to you learning more context about how the mimic got there. Same with the moths and larvae, you could try finding out if they’ve been born and died in this cave? If something made them flee their previous homes (especially “easy” if you have anything to converse with animals), etc. I don’t think it’s the DMs responsibility to give you context unprompted. It’s their responsibility to give you context once you’re looking for context, and even then, you still might need to pass certain rolls to get context. In the end, most of the time, the DM shouldn’t and wouldn’t give you any info your characters wouldn’t have.
This also extends to other areas you complained about. If you want more information but your DM doesn’t immediately provide it, prod them for it through character actions. “What is this Morlock I’m fighting wearing?” “What’s the color of the stones here?” “How high is the ceiling?” “Can I hear anything at all?”
I think you’re on the right track with your Update though. Best of luck!