I agree with everything you're saying here. But over time, some of these things get better. Once you're hitting UNM clan boss you'll get way more books, and as your account progresses, the pressure in this area also decreases (because you'll have a fully booked set of core champs).

Potions almost take care of themselves (again, UNM clan boss and also random rewards from events, DT, etc). The big potions are the only ones I ever run low on. Even though it's annoying to spend 14 energy on a brew, the average rate is actually ~1 superior potion per run, so 2 energy refills to fully ascend a legendary. This is a drop in the bucket compared to...

Minotaur, ugh. This is bad and it doesn't get better. We really need stage 20!

Anyway, if you enjoy the game I'd suggest to hang in there. As your account progresses, the strategic possibilities really open up and the fun increases.

Can be a strange place at times, I personally find it's too congested. You could play the arena and classic hall all day just to stand still. You have days where there's too much to do and others where you find no interest at all. I feel like quiting just because I cannot get past stage 14 of spider but I won't, I'll just move onto another part of the game. I'm also very near to doing the 270 day log in.

Booking gets better with time the longer you play. You will always need to be more picky with Lego champs but eventually rare/epic books will matter much less.

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10 hours a day of screen time might be one of the bigger reasons you are feeling exhausted with the game.  The game is definitely a long term game, not a short term one.  My recommendation is to take a week off to reset.  

By this I mean logging in only to do the daily quests then logging out.  This is effectively what I do every now and then, and the dailies usually only take me a bit in the morning when I have breakfast, and then a bit at night as I'm making dinner.  Sometimes I don't even focus on all the dailies, but rather just the ones that give me energy refills.

Then, when a fragment/fusion event comes on, I spend significantly more time in the game for dungeon runs, champion leveling, etc, and this is where the bulk of my game progression takes place.  Once the event is over, I go back into "hibernation".

A lot of champs don't need to get fully booked, because only some of their skills really need books. Use your books one by one, hope that the random skill that get's increased is the good one and stop booking the champ when the wanted skill is maxed.

A champ like Ghostborn (Dark Elves, lgendary) for example is functional, when his A3 is booked. You cant decide to wich skill the legendary books go, and with bad luck you could have his A1 and A2 maxed before his A3. But if you are lucky, you only need 3 legendary books at all. 

Or Aniri, who was mentioned by you. I didn't fully book her on my account. If you are lucky enough to max out her passive and the A2 (increase buff duration) first, you could stop the use of further books.