I have personally been an early employee at a venture funded start-up that ultimately didn't succeed soon enough at paying customer acquisition and ended up selling intellectual property for only a bit more than it cost to develop it. And, then I took a go at the solo bootstrapped thing like you have been doing. I developed an enterprise type web app for a particular industry and have had potential customers tell me that is really cool and premium but their felt need wasn't sharp enough to make room in the budget and spend meaningful money. So I know what it is to build something and not get to a good market fit with paying customers.
In looking at your situation it would seem critical to understand the backlog of customers that you mention.
Are these customers who have already committed money/signed contracts? Or, would these be considered more like warm prospects?
How long are they having to wait between decision and implementation? Do customers in this backlog know what kind of wait time they are facing under your current solo part-time commitment? Are they willing to wait? Or, are you losing customers to other competitors or internal solutions because of the wait time?
What is the dollar value of your current backlog? If for example it was another $2000 in monthly revenue that is committed but at risk if you don't deliver fast - I would encourage you toward greater commitment try for a small business loan or take on your more personal sources of investment that you mention in option 3. On the other hand if the backlog is something more like $500 in monthly revenue I might suggest that you continue with #1 because your growth curve might already be slowing down.