I may be reading you post  incorrectly as IDK what you mean between "custodian" and "provider" but you still have to have a custodian as the LLC would be owned by your SDIRA as just another asset. 

What I wish I had known when making somewhat similar decisions a decade ago was how to do a professional underwriting. 

I made same decision on the electrical.  

Additionally, working with a local real estate agent or property manager who understands the market can provide valuable insights and help you make informed decisions.

Factor these potential costs into your decision-making process.Prevent Future Issues:To avoid similar issues in the future, make sure to thoroughly screen tenants and emphasize the no-smoking policy during the leasing process.

Hobby landlords primarily make business decisions based on fear.

Is it a “bad” decision to put 20% down on this multiplex, likely have most of the mortgage covered by the rental income while continuing to pay rent at my current apartment? 

Yes this is not what you wanted in home ownership, but don't let a tough situation become a disaster.Take emotion out of the decision and point out the areas that my logic is off base.I wish the best for you.

Allowed me to make a more informed decision versus just relying on the marketing materials from the syndicator. 

What are your criteria for what makes a "good investment" and how do you come to a decision?