You are definitely not the first asking for this feature - there are various posts on this forum discussing possible implementations and their up- and downsides. See e.g. this thread:
The take-away is (imo) that it is very hard to implement selective sync in Syncthing that works for everone in a safe and reliable manner (i.e. that doesn’t eat files and doesn’t confuse the user about what is actually stored where). Schemes based around .stignore in particualr have various edge cases (i.e. what happens when files are renamed or deleted? how do you deal with new files inside ‘selected’ folders?)
The Synctrain iOS/macOS-app implements selective sync by placing ‘selected’ files and folders in .stignore (more details here). This works reasonably well, but really, really depends on proper support from the user interface (which actually prevents certain scenarios, such as selecting files inside an already selected folder, warns the user when deselecting a file that is not present on other peers or when that cannot be verified, alerting the user in various ways when there are new files, integration with other features such as drag/drop and photo sync, etc.). So it is possible, but (as far as I understand) unlikely to make its way into Syncthing core.