As a fellow F2P, I’m prioritizing costumes mostly. Best chance of getting something new. I have a lot of the good costumes that have already been released - Rigard, BT, Kiril, Sonya, Skittleskull, Tiburtus, Melendor, Kasshrek - so I’m waiting for the next round to be released before I pull. If you are missing several of those key ones, I would pull now before the new batch gets released and dilutes the pull. I almost never run Rigard, Melendor, Skittleskull, or Kasshrek out of costume, I switch Tiburtus, Sonya, and Kiril in and out regularly, depending on the need, and although I almost always run classic BT, he’s a game-changer for monk trials as one of the only available healers.
Are you done with S2 normal and hard? If so, you might consider pulling at the S2 portal to get Wilbur and Proteus, since you are missing both. I finished S2 completely, without pulling either, and I saved up for a 10-pull that netted me Wilbur. No regrets. Also pulled Proteus on a lucky single pull later. They are as good as advertised. If you are not done with S2 yet, though, give yourself a shot with those free coins first.
I’d hold off on S3. You still have several more coins available there, even if you are completely up-to-date on provinces, and as others have said, the best heroes there may be yet to come.
Event odds are terrible, as stated. Presently, I’m saving all challenge coins to pull Guardians every five months until I get at least one of Falcon or Jackal, and I save EHTs for Christmas and Sand Empire each year. I feel like those are the best heroes for F2P. I would not spend gems there, though, as the odds are just too long, and if you are patient, you can get a decent number of pulls for free over the course of several months. For example, I did 24 EHT pulls on Sand Empire this year - netted one five-star (Horghall) and one event hero (Gafar).
The classics will come. I don’t think I’d ever target them specifically with gems, unless it was when their costume is featured. After going over a year and a half with no green legendaries, I’ve gotten Horghall, Lianna, and Kaidlen all in the span of less than a month. I say again, the classics will come. (Vivica is pretty great, though!)