Is there a way to only include a subset of performances?

Unfortunately, no.

You will need to clean out the performances you do NOT want in the library and then create a library containing the rest.

The good news is that you probably have perfs you want to save but just don't want them in the library.

So one way to save them is to just create a library of all performances as your backup. That way you are sure to have saved everything those other performances need (waveforms, etc).

Then clean out the perfs you don't want in your real library and create the library you wanted in the first place.

Another way is to create that main library, reset ALL user data, and then IMPORT only the perfs of interest to get the perfs and other objects (waveforms, etc) they need and then create your new library after that load.

Either way you will have to purge the stuff you don't want and then create your desired library.

The Melas tools can do all of that for you if you have that software. It lets you choose what perfs to add to a new library.

Using MODX only, I would first make a full backup to be sure I could get back where I started. Then create a library file of your entire user section (standard library save). Then initialize only your user area so it's empty. Install (load) the library you saved so now what was previously all of your user files are in a library. Then use Library Import to choose which Performances you want to keep from this former user bank library. Now your user bank has only the keepers and you can make another library. This time it's your keepers. Finally, you can delete the library you loaded which has your entire user bank (keepers and throw aways).

There's a tutorial that shows how you can edit Library and Backup files using the John Melas Waveform Editor:
https://www.musicproductionguide.eu/MPG/MusicProductionGuide_2021_01_EN.pdf