I’ve played f2p for 6 months, c2p - vip plus $20/mth for 6 months and am currently vc2p - <$5/mth no vip since April
I can tell you hands down, the best investment for my gems was roster space. As a low spender, you do not want to feed away hard to get heroes. I keep a unique copy of every hero 3-5* and try to max at least one.
3s - I keep copies till I max one, to top off any lagging specials. Then I feed them away except I’ve kept a copy of Kvasir and Bjorn.
4s - I keep one original of all and one dupe of any non tc20 heroes that I have already maxed. I’ve kept a copy of BT and Rigard, in case more ToL format events come up.
5s - I keep all copies in forever optimistic hope of reaching HA10 … one day. Someday.
While most of my gems are used for summons, I agree that it’s probably one of the least satisfying places to pump in gems … unless you like buying feeders for gems. Still … heroes are what make the game … so we persevere.
I’ve used gems to speed up chests in my initial f2p months. Strangely never since then. Never ever used gems to speed up buildings or research. I used to think people are crazy to spend gems on avatars and pins, but, there was a point I was in the verge of quitting and didn’t care about gems and bought an avatar pin. Funnily enough, that’s my most satisfying gem purchase. Go figure
Edit: My first and only 30 pull convinced me of the futility of it for a f2p/ c2p. The opportunity cost is way too high. Most of my big heroes have come from TC 20 and 10 pulls.