Sure it's more work, and takes more time, but the findings will certainly be more consistent than having 5 engineers on it.

In all the videos I've watched and the books I've read, I'm fairly consistently left with the feeling that there always something that they're not mentioning.

I'm assuming your retrofit consists of putting up plywood, holdowns, and clips to existing walls. 

Portfolio consists of 1/2 block comprising 4 upstair/downstair duplexes, a single family home and 8-plex.

Consistency is key.

A direct mail campaign will eventually get you the results you need but consistency is KEY when it comes to any form of marketing.

Trying my best to avoid spending several hundreds dollars on direct mail consistently, which per most advice is the only effective way to do it.

You have to understand they're doing their best in ugly, uncivil situations that require mediation and have real stakes.What they want out of you is consistency in documentation, a willingness to work with tenants, a grown-up attitude, a hesitancy to bring things to court that don't require court to settle.

(this makes my income more consistent and my bookkeeping easier, as well as not having to track which funds I might have to refund in the event of a cancellation, etc)

You have to be consistent.