It seems most of what needed to be said was already mentioned.

From my side I would recommend going for the 30 FG gacha as a F2P player myself, at least as a start.
Once you get an army of 5*, I'd switch to the 50 FG ones.
Don't be too worried about 6* at the start, I've got my first one at 1+ year of playing.

Blooming and promotion should be left for when you're actually feeling wealthy enough to tackle it.
Promotion needs a lot of currency that you don't get quite often, so I'd stick with your favorite girl for this one.
As for blooming, some girls get quite a boost with their third ability, so you might want to concentrate on those, though in the end you don't lose much by just picking your favorite.

I don't know how far Nutaku has gotten with their updates, but I have a "good practice" concerning equipment seeds.
I amass them until an event with additional upgrade exp appears - then I use them all at once.
Last time I checked you can't get forge spirits out of the equipment gacha in Nutaku. Here you can.
I don't use any equipment as fuel for leveling other equipment pieces - in DMM you get enough forge spirits for all your leveling needs.
All the leftover equipment you won't be using can be reforged into bond crystals, which in turn can be used to buy commander medals, which then can be used to buy 5*+ gacha tickets.
Using those, you start gathering rainbow medals. 300 medals and you get a rainbow unit of your choice.
I would avoid using bond crystals/commander medals for anything else, unless you have a surplus of the first - you can get some 5* girls (not in the gacha, though weak) to fill up the team or anything else you need from that specific store.
The mentioned 5* girls can be fed equipment and skill blooms you get by using the special plant pots in your garden (the golden ones).

I'll add you as an ally in a moment.