@Chris Potter congrats on building up your rental portfolio. Sounds like you've been finding great deals.
Just a point of clarification. You can have individual properties with individual loans on them with a portfolio lender. What a portfolio lender is is a bank or credit union that carries the loan themselves. That means it isn't backed by Fannie or Freddie so they are not limited to any specific number of properties. Because they are not backed by Fannie or Freddie, these lenders can be a bit more strict on their qualifications since they are assuming some risk. They also typically have higher rates. Right now, my partner and I have stellar credit and typically hit a 1.2 minimum debt coverage ratio. We can get portfolio loans with our lenders at 20% down, but our rates are between 5.25-6.5% right now. I know it's high, but we analyze our properties with these rates so we work it into our deals up front. 30-25 year amortizations and terms are pretty easy to find as well, but I think the sticking point for you might be the rates.
Adding your wife might be a good option, but at some point you'll grow past 20+ properties. Looks like you're finding deals and that is a good problem to have.
DM or email me if you want to chat more about it. Best of luck.