So I just got my first lvl 40 hero (5*/40+1 Lucina) and I'm happy but it did start to get annoying getting there. To me the game is split into 3 sections when lvling a team. Early stage (1-15), Mid stage (15-25), and End stage (25-40). I usually set up at least 3 teams for the respected stages.
Early stage: This is my 2nd favorite part of the game. While most 5* tend to have some decent skills starting out, 4* and below seem to just have basic stuff until they can build up some sp. While combat is enjoyable at this stage, the moderate amount of 1 shotting opponents and lack of unique skills holds it back. On the plus side because of the low stamina cost for early missions you can be in this stage for a hours (most likely get to 2nd stage before running out). When training during this stage I tend to put my high lvl healer in the party as the exp doesn't drop as much for healers.
Mid stage: This stage is my favorite stage of gameplay. Theres not much 1 shotting opponents unless your hero completely counters them, theres a good variety of skills in play (while keeping most "op" ones out), and Stamina cost seem fair for the experience. This also seems to be where the Ai opponents have a lot more classes (offensive healers, armored tanks, horseback mages, etc). For me at least, this is also where I get the most orbs from story quest and learn most Hero skills.
End stage: I do not like this stage. I'm sure it will get better later on if they add more end game events, but right now I tend to only enjoy arena battles. The Stamina is crazy high and doesnt seem worth it for the exp gained. The Ai seem to be devolve to only Tank, Horseback Mage, or Hero class (character actually from FE). Battles seem to be, either 1 shotting opponents or doing 0 damage. This could be more annoying to me since my main was Lucina equipped with Aether, which requires 5 actions to proc, but I miss the "trading of blows" in the early stages. My Gordin could do 100+ damage in 1 action (he was hitting 4 times) to some opponents and 0 to others, which I get is counters, buffs, and defenses, but it seemed very uneven. Attack, Defense, Resistance seemed to jump way up at this stage while Hp seems even across the board. My Lucina (who I consider a glass cannon with some healing) has 46 hp, to my knowledge I haven't seen opponents over 60 hp. The 46 to 60 may seem like a big difference but when you do 47 damage (not counting a speed bonus) it seems irrelevant. Idk but for me I'd rather do some damage vs the current 0, and just buff everyone's hp respectively to their class.
Thats just my opinion on the gameplay, and I'm sure its skewed based off my core team (Lucina, Sheena, Gordin, Serra), but I would be interested in what other players think. All in all its still one of my favorite mobile games.
Yeah, Imo it seems very unlikely you would get the sp by 40. mine is lvl 22 (ive spent 270 + 111 in pool /so 381 total and have 949 sp needed to unlock everything). Hopefully another lvl 40 merged is enough sp.