You're thinking too small. As I just stated above, the real exploit is to take 3 levels of rogue to get fast hands and then switch over to wizard. Higher level scrolls are expensive, but depending on the campaign you may not have anything else to spend your money on anyway. Cast any spell you scribed a scroll for as a bonus action and then ready to cast whatever other spell you want.
If you really want to exploit things, point out that the limit to 1 spell per round is dependent on casting a spell with a spell slot. Since you aren't using a spell slot when casting the spell off a scroll, it doesn't qualify.
We are talking about a Cantrip here. This is not some huge exploit and Cantrips are generally among the weakest magic items you can use
The entire spell slot thing was put in specifically so you could cast more than one spell turn and it applies with or without Fast Hands. Further Fast Hands will not be the most available or powerful use of using a scroll and a spell. Quicken spell will be - Cast any spell you have scribed on a scroll and then Quicken any other spell you want .... and a Sorcerer both has much higher level spells available and does not even need to use his reaction to do this!
Fast Hands was put in the game specifically to allow a Thief to activate among other things a Magic Item. This will be one of the objectively weakest uses of that ability when it comes to Magic Items, weaker than what Sorcerers can do with Quicken spell and far, far weaker than what other multiclasses can do in a similar fashion.