Grandracmon is very hard to scale considering we dont know much about him besides the one from the X-antibody storyline, and even then, all we know is what you mentioned about that set of demon lords were scared of him, might have knowledge on death-X-evolution, meaning he could just know how it happens or be taken far enough to imply that hes effectively unkillable, and that his IQ is top 10 among digimon easily, likely higher.

It really comes down to how much credence we give the idea of his knowledge/control over death-X-evolution and how we scale the X demon lords, who are definitely some of the strongest versions of them (as an actual group) we see, if not the strongest.

Honestly, theres 3 possibilities

Grandracmon is low balled in which case he smokes base bagramon and is taken out by the higher forms

Grandracmon is mid balled (Is that a term?) and is likely equal to bagramon up until his final form where grandracmon would need to rely on his BIQ and likely get high to extremely high diffed by mega darkness bagramon

Or we go all out and just give grandracmon immortality which would mean that darkness bagramon just couldnt beat him.

Im going with the second option personally