Just make sure you have enough open intake so the tent walls are slightly sucked in with the exhaust fan running. If you want to have an unneeded intake fan then you need one that has as much input as the exhaust outputs allowing for the restriction caused by a carbon filter.
No need to flush when switching to flower. Start feeding the Sensi Bloom and a half dose of Big Bud a week before flipping but don't go nuts. There's going to be lots of nutes left in the soil so you don't want to go overboard tho the plants feed more during the stretch than at any other time. Feeding drops off rapidly as stretch ends.
Yes AN nutes are salt based as are all nutes made for hydro. I've used AN mainly for almost 20 years and and tried the Sensi and Connoisseur a while back but switched back to the 3-part so I have better control over what I feed them.
I just use the base nutes, Big Bud and Rhino Skin from AN, Any CalMag will do and some Epsom Salts are cheap. Overdrive doesn't do anything and the only B vitamin plants need is B1 so I bought some tabs to grind up and add to the mix along with Zinc, Iron and a couple other things. I would be very careful using too many supplements. I've had forum buddies totally destroy big grows tossing the whole line in. A little unsulfured blackstrap molasses will feed the bio-herd and help the plants grow but does nothing to 'sweeten' the bud.
If you have exhaust going all the time then adding CO2 is useless. When I flip to flower I'll start using an alcohol lamp to add lots of CO2 to the grow but then I'm letting the heat get up to 90F with the exhaust fan temp/humidity controller set to allow that temp and much higher humidity before it kicks in. Really does make a difference then but my goal is a sealed room so I can tweak the environment to be perfect all the time. Very low RH here and impossible to keep up with an exhaust fan going all the time. I grow in an underground concrete bunker so if my walls suck in I'm in trouble.
Sounds like you have low RH too so getting a temp/RH controller would be a good idea. Plug the speed controller into that then to your fan. Much better control over conditions that way. If your tent is sitting in a spare room or something you could run a humidifier in that room to raise the RH of the air so that it's at the right RH when it's sucked into the tent. Any moisture added by a humidifier sitting in the tent goes away every time the exhaust is running so your RH goes up and down like a yo-yo. Same with an AC unit.
Decent job for your first grow so it can only get better!