If someone is permanently injured or will have a long recovery time with physical therapy sessions, etc. involved, it's easier than you think to approach that limit.

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.

This is 4% physical vacancy.

Would something physical like mailers or hangers be effective?

I went to school for physics and computer science, then did what most do.... got into a field that was as far from my education as possible. 

This scales your investing.Forces you to network and depend on a team leading to relationships = more businessForces you to develop communication and business systems to stay on trackConsNot as easy to physically be on site if neededReally it comes down to how you want to invest.

When we look physically at structures around American cities it seems obvious that more lost speculating on just cash flow.

What do you think of the many ways to invest in RE, like innovative ways you don't have to be physically involved?

With the schlage you physically have to turn the lock to lock it and that's how I prefer it.

I am renting out rooms in my place, and want to use a digital lock for the front door instead of physical keys.