The cashflow would then be supplemental retirement income for them.

I’d highly recommend trying to add in some small to slightly larger multi family (3-8 units) apartments as buy and holds to supplement your income down the road as you look toward retirement.

It may be different in other states, but I opted for a landlord's policy supplemented with a $1MM umbrella.   

At 2.5 years, we got a supplemental loan, returning $67K. 

Or if it does make sure you select supplemental LDW insurance.

I think my premium has doubled in 6 years, I'm healthy and not elderly or at 65 for SS, so not much longer with that stuff and I'll be getting supplemental policies for SS medicare.

Is spec home building on a small scale too much of a risk as a supplemental income for someone (if they’re qualified for the task) that wants to dabble in a construction building business?

The author describes automating a business, in his case diet supplements.

Having a good plumber, electrician, etc that will work for a little less to supplement their income is great.

A possible/proposed solution might to be supplement the existing posts where there are issues, and add new posts where there may be any risk of moment forces causing failure.As long as there's not acute damage due to water, termites, etc of your joists I think this would be a reasonable approach to at least explore.Most jurisdictions do look for opportunities to ensure that when a contractor touches something they replace MOST or ALL of the system.