I have done 3 showings with a few more lined up. 2 parties have expressed interest and I want to request your help in choosing between the options:Party1: Willing to pay $4,000 p.m.

Dude, you're paying rent to someone.

*As a side note, I think maybe it would be fun also to try and identify where new technology or machine-learning driven approaches could increase efficiency in the spaceAs for my financial situation, personally, I have around 50k saved but 20k of it is tied up in assets I don't want to pay capital gains tax on.

However, even though they both have jobs now, they still do not pay the weekly rent that they promised to pay.

Keep in mind it’s more than likely most of the competitive banks these days for real estate investing may not be local!  

I can't really afford to pay two mortgages.

That is not uncommon, BUT asking the seller to do that likely means paying a bit more for the property.

Just need to stat under the occupancy limit.Also keep in mind basement needs to be livable square footage, ie have egress and proper ceiling height. 

I have a confession: my only duplex, which I gave owned now for 2 years and recently moved out of from a house hack, has proven to cost me more to maintain/pay taxes/repair/fix etc to the point where my cash flow is close to zero.

If one tenant stops paying, there goes 25% of your revenue.