Of course there are countless way to cheat on a budget cap. Yet, I doubt this will happen right and center. We are talking about a sports in the end, and one of the central tenets of a sport is fairness, and I do hope teams do keep that in mind. In the end, a sport in which cheating is common will not be attractive to viewers, so any team that cheats and gets caught does a major disservice to the sports as a whole. Will teams seek out the boundaries of the budget cap? Sure. Will they occasionally cross them and be called back? Most likely so. Will they outright and deliberately cheat? That would be very foolish, and if they do and get caught, they deserve to be penalized in a way that makes Ron Dennis's 100M fine for being a general asshole look like a girls scouts cookie sale.
I don't mind drivers being excluded from the cap that much btw. Good drivers should not be -that- far apart. Give big teams the chance to spend lots of cash on an expensive but proven top-driver, while the pseudo-privateers gamble on the next big shot from F2, and have them grab an occasional championship with a Leclerc-type driver that they subsequently sell to Ferrari for good money*. If anything, it may lead to a bit more dynamics in the driver market.
(*or, since most such drivers are already part of a junior program, pseudoprivateers may be able to land a good upcoming driver cheaply, and occasionally one of them may be sufficiently good to beat the mother team for that one year before moving on to it).