How are people buying multiple non owner occupied properties on fixed rate traditional 30y mortgages? 

The vast majority of "regular" or traditional (Conv/Govt) mortgages are originated by specialty lenders and brokers and are sold in the secondary market to provide yield on bank balance sheets (think Chase or BoA) or to be securitized into MBS.

Are you sticking to traditional banks, exploring private lenders, partnerships, or maybe something creative like seller financing?

This probably implies FIRE in his 20s, but I hope he is passionate about the path he has chosen and excels at it until at least a more traditional retirement age.He started the 2nd rehab he is leading.  

If you're trying to make money without ownership you might want to consider traditional wholesaling by finding discounted properties off-market and then selling to investors.

As we get back to a more traditional market CoC may not matter for people. 

We do traditional PML full set of DOT, note, etc. loan documents for $1,500 for residential. $3,500 for commercial.Hope this helps!

Financing: Would you try creative strategies like seller financing, BRRRR, or syndications, or stick with traditional loans?

If you don't wish to (or have the time to) roll your sleeves up and do some grunt work I would just stick to traditional investing.

If you don't have the downpayment, go make some cash so that you do, either through saving from your normal income or some real estate investing efforts.Your point on debt paydown, although true, is not relevant to the argument of BRRR vs. traditional rental.