You've had a lot of fun in your life, and that is worth something. So is being retired, and I'm glad you're looking at the numbers that could make that possible.
Lake House
I've got to say, if your main house is where you can walk to the beach owning a lake vacation home as well does seem a bit like overkill. But: you say you are emotionally attached to it. You should think hard about whether you are going to regret selling it, but if you keep it then you have to start treating it as a financial as well as emotional asset and rent it out on a commercial basis when you are not there. Mortgage interest at 2.75% is just less than inflation, so not too much of an issue.
Boat
Is ownership of this tied to the main house or the lake house or do you use it at both? How much do you use it, and is selling and occasionally renting an option?
Cars and tiny home trailer
Dear dog. Personally I'd sell everything and start again with two small runabouts. And that's coming from someone who once had a car hand built to her own specification and kept it for 30 years. The internal combustion engine has had its day and there is no long term value in a "special" car other than a piece of art locked away in a garage.
Main House
Yeah, keep it. Maybe think about the ADU for yourself later on when you are too old or disabled to get up the stairs in the main house. But if you really want rental income then start renting out the lake house: it's there and ready to go now rather than being a whole lot of new expenditure that's at least a year off bringing in money.
Education debt
At 2% interest this is costing less than inflation and getting cheaper by the day. Pay it off on schedule and no earlier. Don't use paying it off as an excuse to put off investing.
Living expenses
You've been pretty silent on these, but you are saving about $75k a year out of an after-tax income that's going to be around $300k a year (I used a California tax calculator for that). That puts your monthly expenses at close to $18k. I'm betting that there's a whole lot of fluff and wastage in that sum apart from the cars and houses you've told us about.
Prescription
I think you are looking at this wrong. 1) I think there's a whole lot of "one big sacrifice/sale" plus "one big building project" is your salvation and you don't need to think about anything else to do with saving money ever again. Here's the thing: it's not. And 2) you are all about putting things off: putting off investing until you've paid off the education loan, putting off getting rental income until you've built an ADU. This procrastination is not your friend.
My suggestion is:
1. Itemise all your expenses. Cut out the waste: the unused or poor value subscriptions, the reflexive purchase of expensive options when cheaper is just as good, the waste of utilities, the over-purchase of items that are never used, the throwing away of uneaten food, the unnecessary car journeys. Make sure you get value out of that money you earn. You are currently saving about $6.5k a month: how much can you add to that monthly figure by cutting out the waste in your life? If you are serious about sorting out your finances this is an essential step: there is no point selling stuff and dumping the money into the investments bathtub if you've left the plug out of the bottom.
2. Sell the unnecessary toys. That's most of the cars, the tiny house trailer, perhaps the boat. I bet there's stuff in your house that can be sold too without being missed. Put the money you earn from selling into a nice cheap, safe, index fund. From what you say that should be an instant boost to your investments of about $200k. That's an instant 20% boost to your current investments: not too shabby.
3. Rent out the lake house. This is an instant means of bringing in additional income using an existing asset. Renting it out will also test your emotional attachment to the place: you may well find it easier to sell after its been rented out for a while. If not, it's still there for you in the future.
4. Stay in the main house. You are all happy there and a kid just starting high school doesn't need the disruption of a move. Don't build an ADU: renting out the lake house is the quicker option with fewer downsides.
5. Don't do anything until you've talked it all through with your spouse. You absolutely have to have them on board before you decide anything or do anything, and nothing is going to work without their enthusiastic buy-in. This thread has ideas on how to do that -
https://forum.mrmoneymustache.com/ask-a-mustachian/how-to-convert-your-so-to-mmm-in-50-awesome-steps/Good luck.