To clarify, with flipping you wouldn’t be paying capital gains tax, would be paying ordinary income tax since this is considered active rather than passive income (assuming that you haven’t held for over a year and can’t demonstrate that you bought with intent to rent).
That being said, with $100k profit yet low rents, this still probably is a better flip than rental, and you may be better off selling it and just choosing your next investment differently, with rental in mind, so that it is a place that is cheap enough and can be quickly rehabbed for rental grade, and keep that one, ?? with BRRR. Choose one that cash flows well enough to make it worth keeping as rental.
You appear to be doing very well at this though, so might want to keep flipping as well (impressive that you already have not one but TWO people who want to buy your current project before even listing it).
You indicate that your long term goal is to build a buy and hold portfolio. With your combined skills, one very good way to grow your portfolio can be simplified as buy 2, sell one, rinse and repeat. In the current scenario, you would sell the current project, use the $100 k profit to buy/rehab a house that makes sense as a rental (maybe one similar to the one you got for your daughter?) and rent it out. You can take a mortgage on this if you want, but one small enough so you still have a healthy cash flow. If you can , may be better to just leave it unleveraged and just save the cash flow to add to your war chest as you buy more properties.
You then buy and rehab a second house that makes sense as a flip with the cash that you have left over from your first flip (as well as any money you may have pulled out of the new rental via BRRR, minus what you've set aside for taxes on the first flip). This time you choose a house that makes sense as a flip, and finish it as a flip (upgraded finishes, etc). Sell the flip, invest the profit, pay taxes etc. Depending on how long it takes you to complete these cycles, you will be able to finance your fully paid or lightly leveraged rental portfolio to the tune of 1-2 houses per year.
As stated above, houses in nice B/C neighborhoods make good rentals, lots of blue collar workers who are lifelong renters, and houses in low A/or high B neighborhoods make good flips. You can buy or sell anywhere, but just keep the end goal in mind as you are choosing what and where you buy...
((If you really want to scale you buy the two houses at once with the proceeds from each flip, keep one, sell one, but this likely doesn’t fit well with your scenario of doing the work yourselves.))
hope I was clear and this makes sense...