Keep in mind that, when you consider "broken", you're going by Type/Moon mechanics, where the [i]baseline[/i] of power for a Servant is "can shrug off bullets to the spine". In a game where almost all combat is basically Dragon Ball Z, Archer being able to summon Noble Phantasms at will and use them quite well is fine. In Fate, powers are vaguely-defined and the power level starts at about the same level where Luke Cage starts having to think outside the box.

 In a game like this one, where most characters are at or somewhat above "street-level superhero" and [i]don't[/i] have Rank C Speed, someone like Archer is a godly-powerful monster of a man. Archer!Ilya at the power level described here would be fully capable of going toe-to-toe with many of the characters here and winning by a landslide. Noble Phantasms are serious shit, and being able to bring up even a "mediocre" Phantasm is basically the equivalent of being able to pull a +5 Vorpal Blade straight out of the air at will.

If you're going to keep true to your promise of Ilya having limited prana and only being able to summon up a few Noble Phantasms, I'd probably make a list and actually keep track of them as you go.