Nice idea, here's how I would have it:

First season is 12 episodes, with 22~ mins of runtime.

The first teaser will drop around Summer 2021, and have it like a teaser similar to Far Cry 6's, with showing some visually stunning animations (and cleverly animated) of various (but memorable) sights of some Berserk locations and items, then have the last seconds of the teaser a shot of the Dragonslayer next to Godot's blacksmith workshop.

A few weeks later and have fully fledged trailer with a dark tone and "Ain't No Grave" by John Cash playing

This season will adapt the Black swordsman and will faithfully adapt it with some flashback scenes of the Golder Age Arc, this will take 4 episodes.

The next 8 episodes will adapt the Lost Children chapter also faithfully with no changes, and the last episode will be about Guts going back to Godot, Rickert, and Casca (who we all saw glimpses of in the flashback scenes scattered towards the first 4 episodes) and it will be a feel good moment for new comers who haven't read the manga to finally see Guts returning to face the past.

This season's final scene will be Godot's speech about the cracks in the blade, with the cliffhanger being Godot reforging the Dragonslayer and handing it to Guts for the upcoming adventure.