Is the DC to escape it via Escape Artist still the same (52 + d20)?
The Wiki states that the DC might be too high.
I am also curious about the part with AC for the initial check.
Does an AC of 73 give immunity to all the Bigbys, regardless of Escape Artist ranks, provided at least 1 rank is had in Escape Artist?
Wiki - Escape Artist - Gameplay Notes wrote: Bigby 7-8-9 take the highest of your AC our your escape artist score for the initial check on those spells. So that's d20 + Primary Spell Ability Modifier + Caster level + 12 - 1 ~ 52 +d20 for a CL 30 caster with 11 INT/CHA mod
As for escape artist it's the total score so ranks + DEX mod + bonuses (spell/items), and while they're not common you can buy a bunch of escape artist items at the Avernus shop, going to 60 escape artist is fairly possible.
With complete investment in escape artist bigby are unlikely to work on you. That said it's clear that this is a huge investment for not much, the DCs for escape artist should probably be lower.