I would do some online research into Realtors in your area to see if you can make a connection.As @Michael Dumler pointed out, wholesalers are going to be a more reliable source for consistent off-market properties with margin.

After a few months of trying to work with my long term tenants (who have previously always paid rent on time), I finally asked them to leave for non payment of rent.

I’m in the Greater Seattle area—would paid ads work for this niche?

What I'm really looking for is a simple platform where my tenant can pay rent online rather than send in a physical check every month (their current system...).

Everything you need to learn/know is available online for free.

To make taxes easier we set up a business checking account and had all the rent paid into that account and all the expenses paid out of that account.

Quote from @Josh King: I have 2 rental properties I paid cash for years ago and I have never borrowed against them.

I just paid off another DSCR so pleased to be in this position. 

Combine large inventories of MFH coming online.

@Andrew Moresi I have been looking around, I found a couple promising properties online and am also in contact with a realtor.