Design of low voltage outdoor LED light system
Sounds like a great little project.
Use that DROK 12v to 3.3v converter, and treat each of the 100 light strings as a single 3.3v load, so wire those 100 light strings in parallel with each other.

It may look physically look like 1 string composed of 5 strings in series, but don't let that confuse you.  The strings should be wired in parallel with each other, even if that takes more wire.  Each string has a single 3.2 v+ and Ground or v- connection.  All the 5 strings' v+ should be wired together and connected to the DROK 3.3 V+ output, and all the 5 v- also connected together and to the DROK V-.

To save wire, if each 100 LED string has the v+ and v- connection at only one end, then get some 2 conductor stranded speaker wire and fasten it physically along each string to bring the v+ v- pair to the other end that needs it.  Then the next string can connect to that pair of v+ v-, and so on for each string.

If you want any of the 100 led strings to be physically separate from the others, make a "3.3v extension cord" of the needed length of speaker wire and run it directly from string to the DROK output.

When you add switches, let each switch 1 or more whole 100 LED strings.

If those 300mah batteries would light a string for about 3 hours, each string draws only 100ma.  Your 3A buck converter could theoretically handle 30 such 100 led strings.

Perhaps I'm saying the obvious.  Good Luck!