No challenge, but wondering if I should've done 30 year mortgages on rental properties.

You should be able to get 75% with a 670.. but what will scare some lenders away is if there are a ton of mortgage lates on the report. 

Depending on what type of underwriting method a Mortgage Broker found most suitable for your scenario + your credit score, you could get slightly lower interest rate although 6% is a good number to plug in to be conservative.  

To give you some background, my experience with mortgages in my lifetime is limited to 3 transactions (conventional) related to my past and present personal residences, as well as 1 refinance.

No matter if you form an LLC or not, you get to deduct the same expenses no matter if you itemize or not (interest, mortgage insurance premiums, other costs). 

Ideally we'd like to put 20-25% down with the LLC holding title and the mortgage.

I am, like other, attempting to do a seller financing, but need my private mortgages originated by an originator/mortgage broker.

I would say consider a FHA 203k mortgage as well, so you have the opportunity to buy a distressed property and rehab it to a rentable state with money that isn't coming out of your pocket.

So if they owe $99k on a mortgage and owe $1k in taxes, then I can buy this property all day because I know the ARV will be around $175k.

I have recently closed a deal on a 4-bedroom ( 4 en-suite bedrooms) house in King City, Ontario (This is my primary residence) and I am interested in reducing some of mortgage payments through short-term rental.