Water thoroughly as seedlings put on good growth, but allow soil to dry until it feels barely moist before watering again. It's possible you may see a bloom in three to five years. As you may know, plumeria must reach maturity and develop a blooming hormone in their tips before flowering. I have tried mixing dry sand with the seed and sowing just a small pinch of them by hand, but the seed always come up too thick. Thinning the plants by hand is time-consuming and tedious.
Seed can be as fine as dust, and it is surprising that some tiny seed produce enormous plants.
Begonias and orchids have dustlike seed. Some seed, such as carrots, can be purchased in seed tapes, resulting in uniform spacing. You can even make your own seed tapes using a biodegradable adhesive and paper. Sowing tiny seed by hand, as you know, usually results in the need to thin. It is a good idea to disperse small seed with clean, dry sand.