Can’t play Xbox Series X Optimized Games online but regular Xbox One games run fine online

The games on the SSD will run fine on it. You won't need to move them to the Xbox Series X storage to play them. Only the Xbox Series X games & games optimized for Series X will need to be played on the consoles own storage.

It's only on the Series S/X that they can't be played on a regular hard drive. The hard drive is too slow to run the games properly. The One is fully capable of running all games from the internal or any external hard drive. The One wouldn't have been playing optimized games at the full S/X optimization due to hardware limitations.

If he has a Series S/X and is trying to play a Series S/X optimized game, it will only run on the internal drive or on an expansion card made for the Series S/X. It won't run on a regular external hard drive or SSD.

Yes you can. The expansion card is for Series S/X optimized games. So you should be able to run any game from them as opposed to regular external hard drives which cannot run Series S/X optimized games.

For Series S/X optimized games, you can only play them off the internal drive or one of the expansion cards that were created specifically for the console. They will not run off of a regular hard drive or SSD.