Are Pokemon Sword and Shield: Brilliant Stars booster boxes a good investment for the long term… the short answer is probably.
A good number of booster boxes are set to gain in value over the long term, 5+ years, simply due to supply steadily becoming more and more restricted. In addition Brilliant Stars has multiple chase cards that give it a bit more value.
Brilliant Stars is also one of the top rated sets from the Sword and Shield era.
An Alternative to Sword and Shield Brilliant Stars is Sword and Shield Lost Origin. Lost Origin is the fan favorite for gaining in value over the long run due to its chase cards and restricted supply.
Either way, both Lost Origin and Brilliant Stars seem to be good long term investments with restricted supply and quality chase cards.