The biggest problem with this spell being used on players is the ability of a DM to rationalize what's considered reasonable.
So I have a solution you may not like, but ultimately ends all argument on it, well at least it did in every campaign I've played with this spell.
The spell requires a saving throw, if you fail it, you are under the effects of the spell. If the DM doesn't know HOW to phrase the suggestion to make it sound reasonable, then the DM can simply state the desired outcome, and let you rationalize it yourself.
In your provided example, the DM could have said this:
"Ok, the enemy caster compels you with suggestion to silence the other enemy caster. You failed the save so his suggestion sounds reasonable to you."
That's it. No roleplay required or rule lawyering specifics on what sounds reasonable. You put the WHY it's reasonable into the hands of the player. They get to internalize what would make that a reasonable suggestion.
If the player says, "What does he say to make me think it's reasonable?" then the DM can counter with, "That's up to you. Whatever it is, you're convinced that it's a reasonable suggestion, so role play it accordingly. The desired outcome is that you are silencing the enemy mage."
The obvious problem with this: If your DM is abusive, he/she may use this to present unreasonable suggestions. That being said, if your DM is abusive you likely already have far more pressing problems than this singular spell anyways. If the outcome doesn't seem unreasonable, then the specifics on rule lawyering should be left up to the player so they don't feel their player agency is being intruded on.
The focus here is the outcome. Instead of worrying about whether or not the question sounds reasonable, focus on the outcome. Is it possible that it's a reasonable outcome? If it is, then this is why this approach works. You let the player rationalize it however they see fit. If the outcome is entirely unreasonable, that should be evident. Like asking a blacksmith to give you his wares for free. That is entirely unreasonable. Whereas asking said blacksmith for a discount is entirely reasonable, and the outcome of the spell doesn't need to be decided by whether or not a player instead of their character is able to vocalize that appropriately. Remember, you're playing a character, and their stats are not your stats. That's why my 20 Charisma Bard relies on roles for his persuasion and diplomacy checks. Because he's far better at it than I ever will be.