I would say he links the worm (that has given him his freedom, even if short lived). He is getting the same type of dreams Tav is and the promises of more power. He craves the power and freedom. As it stands the party is assuming the Absolute and the worm are connected. Get the brand get more from the Absolute…
No, I understand your direction completely. But... the way I see him, the trauma from his last only seemingly harmless tattoo still haunts him by the daily.
You can RP a lot. It's just that extra factor of disbelief you need to get over. I'm also struggling with companions just knowing being branded will let them use the seemingly baned equipment. Without being really aknowledged by the absolute?
If I had to play it straight, I'd get the mark to pass better disguised among the absolute's minions. Believing Halsin will cure us soon. Then figure out the gear doesn't look as ominous as before...
I usually have given Astarion the book of Thay by that point, though. Maybe the darker story beat may be started here, encouraging him to commit fully to the tadpole...
Altogether it's a very Wyll in proportions of low wisdom thing to do. I guess if your MC is high charisma, through /incredibly/ stupid, you may convince the full party to run around with the warboard lmao