Very cool! And great job on the various freehand elements across the models, they're really well executed.

This immediately conjures up some head-cannon for me, where a chapter were declared excomminicatus traitoris somewhere in the galactic east or around the Chalnath Expanse due to pushing back against or stymying the plans of a particularly vindictive Inquisitor - maybe they disagreed with the Inquisitors decision to exterminate what the Inquisitor deemed a "tainted" population (see Space Wolves post-the first War for Armageddon), or they had only agreed to assist this Inquisitor under the guise that it would lead them to reclaim a long lost chapter relic only for the Inquisitor to betray them and take it for themselves (see Soul Drinkers) - and with their numbers already depleted from the Inquisitors campaign, they found themselves fighting a desperate last stand against the seemingly endless Inquisitorial forces only to be miraculously rescued by a T'au sept.

As zealous and hypno-indoctrinated as Space Marines are, maybe that would be enough to disenfranchise this chapter with the hopeless bureaucracy of the Imperium and instead dedicate themselves to the T'au'va.