I think it would be beneficial to bring in a veteran QB. At least just for training camp and preseason.
He should play a lot in the preseason.
I have come to realize that the veteran QB idea is often overrated, especially in this situation.
The Bears have several coaches now in this new offensive coaching staff that are going to work with Williams - you have Shane Waldron (OC), Thomas Brown (Passing Game Coordinator), Kerry Joseph (QB coach), and Ryan Griffin (Asst. QB&WR) all carrying titles from OC to Assistant QB coach that have varying levels of NFL playing and coaching experience to give Williams all the coaching he may need.
I do not see how adding a likely journeyman QB to the mix adds much value of anything. That journeyman QB is also going to have to learn the offense and just takes away reps from Williams and/or Bagent. And worst case, it adds a voice to the mix that doesn't really help and just adds unneeded thoughts to your young QBs that you are trying to develop. I remember Fields commenting last year that there were too many voices and it made him robotic.
I go back to you have 4 coaches on this staff now that will have some hand in developing the QB position. If that is not enough or they don't know how to coach and develop a QB, then the bigger issue is you hired the wrong damn coaches and signing a journeyman qb doesn't fix that.
It would be different if Williams was joining an established offense with an established veteran that had been operating in that offense. In that scenario, there would more likely be something that Williams could learn and maybe you could consider slow playing it a little bit. But, that just isn't the case here. So imo, it's a waste of time and reps to not just acknowledge Williams will be the day 1 starter and give him all the reps with the 1's and maybe even some with the 2's.