I hope this is not the case, that Apple realizes the blunder that it has made and returns the Retouch feature to Photos in the next update. (How hard could that be?).
So far, the only workaround that I have been able to use with any success is to upload and edit my images in Photos on an older Mac, and then export them to my Studio. I am seriously considering buying a Mac Mini to use just for Photos running a pre-15.1 version of MacOS. I have warned every photographer I know who has not already done so not to upgrade to 15.1.
I tried Pixelmator but did not like it at all. I then saw that Apple just purchased Pixelmator! If Apple intends to incorporate some of Pixelmator's features into Photos it will be a complete disaster.
I will give Apple another month or so to restore Retouch but if this misstep is not rectified by then it seems my only option will be to abandon Photos entirely and switch to Lightroom. I have over 10,000 images in Photos so this will be a colossal undertaking. Unfortunately, unless you are OK with storing your pics on Adobe's server in the cloud, the only option is Lightroom Classic, which has the necessary feature set but its interface is outdated and a bit clunky.
Apple Intelligence, at least when it comes to Photos, appears to be an oxymoron. As a long-time (+30 years) Apple user, I am extremely disappointed.