I've been trying to make Excalibur work for months now, and seem to keep hitting issues, usually either doing middling/pathetic damage or just losing survivability almost entirely to try and fix that. Kullervo's subsume helps a lot with damage, but his base kit is just not good enough to tank steelpath if you want to build into strength whatsoever. Gloom feels like his best helminth option, but he still dies semi regularly if you aren't actively trying to shieldgate, which again kills your damage output.
I feel like all he needs is a buff to his 4, and either a rework or replacement to his 1 or 3 that gives some kind of strong survivability tech and he'd be alright. Radial blind is good as is, but it isn't enough to keep him alive, like, at all by itself.
Those two things would probably be enough to make him viable, but some stance combo changes (like the slide mentioned), and maybe building in stuff like chromatic blade would help a lot. The fact he basically needs that augment sucks, and he already has strong competition for his mod slots.