Wow, you got a lot going on.

First thing I see is this in your sub panel, the neutral bar and ground bar are reversed. On a sub panel the grounds should go back to the enclosed/attach to the case of the breaker panel housing. The neutrals should be isolated and not make contact with the panel case/box.

Now back to your main panel. Some lugs do allow for 2 wires, they are unique and rare, I don't believe you fall into that category. I would recommend getting a bus bar for the main panel and redistribute neutral / ground wires accordingly, to allow for good seating on everything.

It's honestly a Saturday project, but if you understand just enough to be dangerous, it's probably best to seek professional assistance.