Cross-save allows you to use a single set of save data, across multiple platforms. In your case, you are using your PSN data on both PS4 and Steam. Those characters are permanently tied to your PSN account, and cross-save just lets you access the save data while on Steam. If you were to switch your active account from PSN to Steam, you would lose all access to those characters and progress. As the PSN save data would be made dormant, and you’d only see whatever data, or lack thereof, you have on Steam. You will want to leave cross-save setup as it is, with your PSN account as main. Purchasing the new expansion on Steam is exactly what you will want to do, if your intention is to play that new content on Steam. Not buying Beyond Light for PS4 is not a problem, if you do not plan on playing that content on PS4. Buy the expansions for where you want to play, and not where the save data originated from.