At the time, I had determined it was not a cost effective solution (even though I was paying myself the interest) once you considered the CMHC insurance premium, the originating fees, and the custodial/servicing fees.    

For example some custodians will have tiered pricing based on account value and how many assets are held under custody.

They also do entity setup for SDIRA entities in addition to custodial services so you have everything in one place.

I am 24 years old, always a bit short on cash and I have full custody of my 2 year old daughter so I would like to keep moving and live-in flips to a minimum.

This eliminates the paperwork, processing delays and per-transaction fees that come with the custodial model. 

Compared to an IRA, Solo 401k contributions limits are roughly ten times higher.There is no custodial requirement for the 401k.You don't need the additional expense and administration of an LLC to have checkbook control.There is a built in-Roth component whereas IRAs are either traditional or Roth, not both.A spouse can also participate in the same Solo 401k plan.The Solo 401k has additional tax benefits over an IRA when investing into real estate using leverage.The penalties for prohibited transactions are less severe, though it's best not to utilize this benefit :)

It is part of the divorce agreement and custody that he makes half the payments.  

Paying couple hundred dollars a year for a vehicle that can generate millions of dollars for you over next 20+ years is pocket change and would be significantly less compared to IRA custodial account. 

They set up a custodial account, then my funds were transferred to an account (that I opened at a local bank) owned by my SD-IRA owned LLC.

If you discuss this topic with someone knowledgeable (not just a custodial sales rep), you should be able to get a handle on your options.