Read further into your source to see if the 'cutting more than 1/3 into the rafter' applies to at the support or if it applies to between the supports.

I appreciate the input, thank you both. Good point about between the supports vs at the support.

The drawing you have there shows the rafters notched for the sheeting. I plan to notch the sheeting for the rafters such that I can tie in the entire bird board into the sheeting.

I am not going to use a ridge board, instead I plan to lay rafters out on a jig and use collar ties, cut birdsmouths and set the assembled "trusses" off to the side. Build walls, boom-lift assembled rafter sections onto walls.

I do not suspect that I will have any appreciable load on the overhang, the snow piles up here in Alaska but not so much on a 12/12 roof.

The more I think of it, I believe I will be ok with a 5.5" birdsmouth but I'm just trying not to screw anything up here. I am not an experienced builder and like to provide people with more experience the chance to correct me.

Options should be explored with more drawings. Include everything in your drawings now while it's on paper/screen. Facia vertical or angled, soffit with support, insulation amount and type including deck insulation, sealing for structure airtightness. No surprises.

I had what was originally a covered porch on slab foundation finished as living quarters. There's a 2" stepdown as you go from original quarters to addition but walls do not look taller. I didn't pay attention to how they framed it but I would assume they added top plate thickness. You could put 3 2x on top plate and use crown mold to conceal gap normal drywall or paneling doesn't cover.

"2x12 rafters, 12/12 slope, cathedral ceiling"

"I am not going to use a ridge board, instead I plan to lay rafters out on a jig and use collar ties"

Please reconcile your roof structural framing to include either a ridge beam and posts to foundation (to retain a cathedral ceiling), or rafter ties (which can be left exposed within a cathedral ceiling). Otherwise, your roof will push out your walls. Also please review online literature of what collar ties vs rafter ties perform functionally (e.g. JLC Online or FineHomebuilding have good diagrams).

Then come back about the birdsmouth.

This seal and insulation info is interesting. The Risinger Build Episode: What's an Over Roof? (youtube.com)