So i noticed you can just click sneak attack and get your sneak attack damage on your first attack.(you generate your own "threatened" near melee)
I also noticed the extra "bonus action on thief"

So accuracy in 5th edition is very low. so linking the sneak attack to the primary attack takes away a great deal of sneak attacks effectiveness.
As in raw any attack you land in a round can trigger sneak attack but only one time total in that round.

In one of my other threads i ran some parse data to show how damage works in 5th ed when compared with accuracy and median DC/AC numbers.

you are basically nerfing thief damage by around 30% by forcing them to use their main hand to hit with it, rather than it proccing or asking you if you want to use it on a successful hit that meets requirements.

However it depends on how deep you want to go into this "extra bonus action". If rogue is going to be picking up more extra attacks instead i say go for it, when i parsed the data the only way to keep rogue in its lane was to add the attribute modifier to each sneak attack dice.
Its also important to note in raw many players choose to use throw with their offhand and at present that is not an option in your game.(typically with magic stone, darts, ect.) throw is only available as a full action not a bonus action

which is essentially similar as adding an extra attack at full acc and damage. Not quite as good because it loses some of its punch on lost damage from misses.

Honestly though rogue competes really well if you just check out my other thread on "A Polite suggestion for monk."

it really highlights some of the key issues that 5th edition has before you even got your mitts on it. These issues will be much more apparent in a video game than a tabletop, as a GM can react to the situations 5th ed presents better than a scripted game.

you will save yourself big headaches when designing this game if you just do an "accuracy check" to determine an abilities actual effectiveness in the system as most of the time any ability that requires a successful hit is about 50% as effective as it looks in 5th edition. you wil need the technology programmed in for things like smite anyway which only proc and spend the spell on a successful hit.

I would love to help you out in any way that i can, i have a great interest in game design and have provided well put together suggestions for many other games even including video break downs of how to fix given issues.
I've been playing dungeons and dragons for almost 30 years as well as many other tabletop rpg properties, i played baldurs gate 1 and 2 as well as some icewind dale and neverwinter nights. I love your work with divinity OS and OS2 and am excited to see where baldurs gate 3 goes during early access.