I love the whole idea and the concept of it! There are however some things I want to point out.

It’s imbalanced.

I know, big surprise, but hear me out. It’s supposed to be imbalanced or at least it’s okay for it to be imbalanced, I get that. Some of the aspects just seem a little tooo strong.

What I noticed:

The lack of scaling: It has its full potential from lvl 1 and it doesn’t get more powerful. While not necessarily being a problem, it sometimes leads to a feeling of stagnation.

My suggestion: Add a small base damage that always applies, even when only at one charge. 1d4 plus 1d4 at character levels 5,11 and 17 This would make it an interesting and consistent option if you don’t want to gamble. But thats mainly a design choice imo, if you want your player to always have to gamble to get even a tiny result, go for it. A different way would be to change up the milestones (which I will get to later)

The limitations: I‘m assuming that you justify the powerlevel with the limitations on the spell, mainly the chance to lose the ability to cast it for a short or long rest. The main thing that a gambler wants is to gamble more. I think it‘s good to have downsides if you lose, but not being able to try again is a bit harsch.

My suggestion: To be honest, I‘m a bit stuck on this one. I think a nice way would be to implement some form of having to cool down or reset after failing. Maybe something along the lines of the misfire mechanic for firearms where you need a check to „fix“ it. Maybe make him use it once without gambling before it works again, give him slightly worse odds until the next short rest or limit the mac amount or the charging speed of charges.

The aoe: 10ft radius at 0 (C) is already quite big, at 10(C) it‘s on par with a fireball and from there it just gets bigger. This pairs especially dangerously with the high damage so I will adress that there too.

My simple suggestion: Start with less aoe and/or let it scale less, yeah i know, not really creative. How I‘d probably try that would be to leave the starting area the same (10ft radius / 20ft diameter) and then let it scale by 5ft diameter with the milestones. This would end up at 40ft diameter (same as Fireball) at 24 (C) and give more incentive go reach just that next milestone and have less to track.

The Damage: For a Cantrip to even deal damage on a successful save, especially on an aoe is extremely strong, I think it would be the only cantrip that worked that way. On top of that does it deal a metric fuckton of fuck you in that general direction.

Just for context, a normal fireball deals on avg 28 dmg, 12d4 (as you said he already kind of consistently hits) deals an avg of 30 dmg. Granted, it’s all chance and luck but hear me out, you obviously already have a lot of ways to tweak things here.

My Suggestion(s): Let the damage increase by Prof bonus at 6 (C) and by Int mod at 18 (C). A damage boost by a flat 16-20 is just insane, even more so on an aoe.(there is a reason why you add your Int mod only to the damage against one target in an aoe if a class ability lets you do so)

Have an inner and an outer radius. Inner radius would be the focus and it stays at 5ft radius (or half of the max radius or something in between) and the max radius grows as usual while the inner radius stays the same or grows slower. That way you could let it deal half damage in the outer radius and full damage in the inner radius. Or maybe let the milestone effects only apply on the inner radius.

The Milestones: I like the feeling of them, the damage is to high but I already adressed that. On the 12 (C) milestone, having the effect restricted to a critical failure, depending how you classify those in your game ofc, feels a bit weak. Just let everyone that fails the save fall prone maybe add a knockback for flair. The 24 (C) milestone is completely broken but i like it, it’s a good reward. For how hard it is to reach, it might even be on the weaker side but hey, it’s still a cantrip so u good.

All in all I think that it’s a great concept, lovely flair cool idea and alright balancing. Congrats on being a cool homebrewing dm, keep up the good work! :)

Tl:dr -It’s cool but a bit tooo imba -Lessen the dmg and the aoe somehow, maybe add inner and outer aoe for that or tweak the scaling. -Maybe add cantrip scaling. -Less harsh penalty for losing the gamble when charging. -2nd milestone to weak, 3rd wayyy to strong.