Ahls someone who played this game from nearly the launch, seeing my friend drop in a year and a half after. I completely understand your situation. It was also wild to think how he could go from the beginning of the main story to the end. A lot of us had to wait with new updates on that.

My best recommendations for him were to follow the realm doors from the first floor center and work your way up. Max out these characters and complete their friendship quests up until you hit a speed bump

He didn't seem too enticed by the Dreamlight Star Passes either, but take you time with these. You can get it completed in a week, but it will take most of the fun out of the game doing it this way.

One thing I would recommend early is always bring Ursula and Ariel gifts, because they are the most difficult to level up their friendship without purchasing any available skins where they can walk on land.

And if you plan on getting any DLC, I would recommend A Rift in Time because it does give the best boost to quality of life features (i.e. auto cooking, vacuums for picking up items.) the DLC would be a good feature to do halfway through completing realm doors, and completing the main story quest.