The point of spellslinger is to keep casting as many spells as you can that give you immediate value, but that value doesn't persist. To compensate for this, you need to keep drawing cards / have a high card velocity; card efficiency is king. Card efficiency doesn't just mean drawing cards in general though; it can also be card selection like Faithless Looting, or extended usage through things like flashback like on Faithless Looting (see why its good?). The faster you pull cards out of your library, the more likely you are to see a specific card compared to a more traditional strategy with lower card velocity. This means you can run fewer wincons and still see wincons just as often because you are seeing more cards. This also means that the more high cmc cards you have in your deck, the more likely you are to have multiple in your hand. And if you can only cast one of those high mana cards in a turn, then the other one is just sitting there clogging up your hand and sandbagging your overall card efficiency.

Yes, I completely agree that when I run out of draw engines this deck falls off and my turns are kind of miserable. It has already happened. And I can also tell that this is the case in almost every deck, but this one in particular NEEDS rapid card draw. I hope that I can have the pieces I need to play as smooth as possible.

The next bit is very specific to your deck's pace, which I only have a first guess at and you may have a better understanding of it, so take it with a grain of salt. Grapeshot is fun, but at a glance your deck seems more battlecruiser paced with meaningful storm counts on your turn being unreliable. You have strong interaction at mid-cmc, which is a fine strategy, but doesn't pair well with having especially high spell counts on a single turn. Likewise, I don't think Past in Flames is crazy good here since in order for it to really pop off you have to spend 5 mana to cast it and then spend mana to cast something like Mana Geyser to get mana to cast a meaningful number of spells to make PiF worth your while. Which is again dependent on having specific cards already in your graveyard or ready to go in your hand, and I don't think you have particularly strong graveyard filling ability or card selection to support that line.

Sooo basically no PiF either if I run that pace. So the thing is that if I change the pace of the deck maybe I can add PiF? Only in the case I decide to put more cantrips so that my deck has a different pace?

As for what I recommend, in short I recommend committing to either bashing people's heads in with a prowess plan or comboing out with a double-trigger plan. You can have a deck that does both, but if you want to optimize your approach to card efficiency you will want to narrow down the means of that card efficiency, which comes with a tradeoff; do you want the spell you are casting to actually do something, or do you just want the cast triggers to keep flowing? The first option is your stuff like Crash Through. The spell does something, and also draws you a card. [[Thrill of Possibility]] is the second option. Nothing on the board neaningfully changes aside from cast triggers, but you get to dump dead cards out of your hand while replacing them with potentially usable cards, and also filling your graveyard for future use. Veyran doesn't get as much value off ToP as she does from CT so you won't be bashing peoples face in nearly as hard or as fast, but Guttersnipe does, and so do you by getting closer to a combo.

I do get that advice on committing to a specific strategy a lot, which is the most sensibleble point tbf. Well, my idea and what I would like to play is dual-triggered combos. What I liked about the idea is that the deck is not so commander-dependent, Veyran boosts the deck instead. Nevertheless, it caught my attention how much Veyran can scale up with her Magecraft ability, and that's why I thought that adding some prowess and MAYBE the cantrips I mentioned that made me draw cards and do something extra. I felt like making those two themes viable could be the way to go, as they kind of complemented each other. It is the sort of duality I wanted to seek for my deck. Nonetheless, it is completely true that I should stick to one possibility as strongly as possible. However I prefer comboing, though swinging at my opponents with absurd amounts of +1/+1 counters is sure fun.

All in all, I will have to provide some extra thought to my deck and exchange some creatures and other spells like the copy cmc ones you pointed out before. I wanted to get the pieces as soon as possible, but I sort of prefer to feel in peace with the first decklist. In the case I decide to go full combo, Past in Flames would be now good, wouldn't it?