This is about understanding the rules and finding a balance for fun.
House ruing to make something more powerful is usually never the right way. The dm should just explicitly balance it.
Given that people have seen the DM but we really don't know much more. I'd really like to see the description for something that might be iconic for a fast-hands Thief. We know the public "uni" items
clearly describe taking a magic action:
cloak: While wearing the cloak, you can take a
Magic action to pull its hood over your head and expend 1 charge to give yourself the
Invisible condition for 1 hour.
club: As a
Magic action, you can strike the weapon against a hard surface to create a loud clap of thunder
As a
Magic action, you can strike the weapon against the ground to create an intense seismic disturbance
bow: As a
Magic action, you can fire one energy arrow from this weapon at a target you can see within 60 feet of yourself.
As a
Magic action, you can loose a flurry of energy arrows from this weapon at a wall
And then you have some other items that do not use that phrase. This not an accident.
mace: While holding the Mace, you can expend 1 of its charges to cast
Summon Celestial (+9 to hit with spell attacks).
and enspelled weapons: While holding the weapon, you can expend 1 charge to cast its spell
and scrolls: You can read the scroll and cast the spell ...
Until I understand why the descriptions are seeming taken from two different molds, one clearly stating "as a magic action" and the other "you cast a spell" ... I will assume it is specifically to exclude fast-hands. I'd love to see the description for a wand of magic missiles ... That seems iconic to me as a rogue tool. And the abuse is not from a rogue, it is from a caster reliably sending off two powerful spells per round. Quicken has significant level limits
As a DM, I'd be sure to balance-up the thief with specific items for them. I'd have items that say they work -- I'd probably invent more interesting items for them.. why not be able to activate a true-sight crossbow from 30 away to setup advantage in an ambush (or to stage an assassination and frame a bystander) .
As a player, ask your dm if you can expect to find appropriate items that help keep this sub-class on par with the others.. because you think it would be fun to play it that way. If the DM is going to make you craft items that are not really unbalanced but bend the rules ... I'd play a different class.