Don't change the feature, change your attack mode
This is a bit of a frame challenge, but I think Umbral Sight can still work in such a campaign.
All creatures with blindsight only can use it in a limited radius, and very nearly all have 60 feet or less (only very unusual ones have 120 feet or more, like elder brains, the Tarrasque or some demon lords).
If you use ranged attacks rather than melee attacks, you will often be able to stay out of blindsight and enjoy the benefits of Umbral Sight. In partciular if you have natural darkvision, as you will then have darkvision out to 90 feet and still will be able to see them.
Addendum: while the above will help against opponents that have blindsight, what you are dealing with is truesight 120 feet, so that will not work.
In this case, I think the most balanced approach to this would be to ask your DM if you can replace the 3rd level feature against another Ranger subclass 3rd level feature. As the subclasses from Xanathar's have two such features (compared to the only one feature in the PHB), to keep it balanced maybe best one of those subclasses, for example the Monster Slayer has Hunters' Sense, or Slayer's Prey, both of which still would fit thematically with the Gloom Stalkers outlook of "seeking to ambush threats before they can reach the broader world".