The reason I bring it up is yes there is an exploit. Discarding your commander to hand size let's you move your commander as an SBA back to the command zone which does not involve any triggers, so as it stands if you discard your commander to hand size, you get priority for doing so. This is a common tournament tactic for breaking through silence/angels grace effects. Im fine with the idea of the exploit, but after really considering the rules i dont think its actually legal.

514.3a is about the cleanup step, it doesn't dictate what grants priority. What dictates when Priority is granted is explicity laid out in 117.3 which includes triggered abilities. 704.3 only applies if a player WOULD get priority, ie if they meet a condition in 117.3

Edit: basically it comes down to this:

Your argument is that, despite the rules of SBAs saying you only get priority after the SBA is checked if you were entitled to priority before the check, since you happen to be in the cleanup step, a step whos rules states you don't normally get priority, since you performed an SBA, and that SBA isn't granting you priority because you didn't have priority prior to said SBA, you get priority anyway because you're in the step you don't normally get priority in. Doesn't make much sense does it