5e is showing it's limitations, they didn't want a character progression path to be tied to feats because feats are a variant rule. You're better off purchasing Keith Baker's Exploring Eberron on DM's guild than using Rising from the Last War's rules. There are rules for Siberys dragonmarks in there. I also thought the Kalashtar and Warforged rules were way better in Wayfinder's guide.
As many have already said, WotC wanted to avoid making feats mandatory for the character progression of Dragonmarked characters.
Exploring Eberron also simplifies the Dragonmarked progression, merging Greater and Siberys Dragonmarks into a single Supernatural Gift akin to Wildemount's Hollow One and Theros's Heroic Feats. Characters from both marked and unmarked subraces of Dragonmarked races can take the Siberys Dragonmark Supernatural Gift; for an already Dragonmarked character, it represents the Greater Dragonmark from past editions, while for unmarked characters, it represents a true Siberys Dragonmark. Whether a character gains this gift or not is entirely up to DM discretion, but the book does recommend giving it out at around 12th level.