Working with an investor in Maryland who has multiple units for sale, could do seller financing on just one or bring in another person outside my LLC to try to buy everything to get the best discount possible.

However, I would really like it if a lender could use my brokerage account as my “skin-in-the-game”, vs putting 10-30% down on the deal in cash.

You'll need to either call to small banks for this type of a loan or work with a mortgage broker - specifically, a loan officer that works with multiple banks. 

I personally have multiple plans-401k, multiple Roth and traditional IRAs, HSA, and several CESAs.

This may be something to consider as a multiple property owner.

You could sell one house and buy multiple homes if you want. 

Simple loans are good for someone early in their career but if you own multiple properties or something like that your loan might cause him some trouble. 

I don't want the offer to be contingent on the closing of my other place as I think that this property will have multiple offers and I don't want mine to look weak. 

I put in multiple offers on 5 separate properties this year and got out bidded on all.

Things get ugly from there.In my early days, I used to accept multiple unrelated tenants as long as it didn't violate my occupancy limits of two adults per bedroom.