If Werner and Ziyech can be moved on and a quality forward can't be brought in to replace them, I would just stick with the current options.
Havertz, Broja, Sterling, Mount, Pulisic and CHO for three places in the team would be more than enough depth. I'd be a little bit worried about the quality but if I were Tuchel and had these options to choose from I would ideally look into building a good chemistry between Sterling-Havertz-Mount starting as many games as possible together and CHO-Broja-Pulisic as their backups.
I'd say one of the reasons for our attacking players constantly underperforming, aside from a certain lack of quality for some, is the constant rotation and players never seemingly developing a good on-field relationship with each other due to the chopping and changing. Since the summer of 2020 there has very rarely been a continuous run of games using the same front line without at least one of the players being changed for another between games. Building a bit of momentum using the same lineup more regularly could mark a big improvement.
Last season there were 7 players competing for three spots (Lukaku, Havertz, Mount, Werner, Ziyech, Pulisic, CHO) and even 8 if you count Barkley among them. Only Mount played 2000 league minutes or more and the other six averaged less than 1400 minutes across the league season, which amounts to only around fifteen full 90 minute games. That's not enough to build any sort of form because as soon as you have one bad game, you're out of the team for the foreseeable.
Having good depth is often considered a positive but having too many players of similar quality competing for the same places can quickly turn into a negative as well. In addition to never developing good chemistry together, players who always need to worry about being axed for the next game can get more scared of making mistakes and perhaps unknowingly start playing too cautious.
I refuse to believe Chelsea's forwards are as bad as they sometimes seem, and the reason for their undeperformance is not only down to the manager's preferred system either.