Creating a large 18650 Array
Advice:
1) Your battery bank should be 48V (otherwise wire diameters will become unreasonable). Run through the calculations and you will see what I mean. For example, if you need, even momentarily, 50 Amps of 120V AC, that would be over 500 Amps at 12V.

2) Your charge controller must be designed for lithium batteries (or be programmable). Lithium batteries cannot be safely floated. You must stop charging them once charge is complete.

3) Investigate any building code issues prior to purchasing anything. If you are going to pull permits to do this, you will need approved plans. An inspector might not allow you to put in a large lithium ion battery pack without UL approvals or some such. If you live in the US you should be concerned about this. If you are outside the US, you are outside my experience.

4) When you put all those batteries together and charge or discharge them rapidly, the batteries in the middle will be unable to dissipate heat effectively. I don't know how big a problem this is but I would be very worried about it. I think you will need a thermal sensor near the middle of the pack to make sure the batteries do not overheat.

5) You cannot put the batteries in parallel without some type of over-current protection. It could be as simple as a fuse or PTC. The reason is, you don't want one bad cell (imagine it failing short circuit) to become a sink for all the other cells in parallel. This could make a bad situation much worse.

That is all I can think of at the moment. I can tell you right now that I would not do this. I would use AGM lead acid batteries. If you provide individual over voltage protection for each cell, that might help prevent the dangers of imbalance. When one cell in a series string stops accepting current, the whole series string stops. So the string would only be as good as the weakest cell.