aye there Garabaldy
I presume the plywood that you are trying to source is for your next build the Fireboat 39?
6mm thickness plywood would be ample for the frames, as this Model is 40 inch length x approximately 9 ¼ inch beam and I should imagine it may have about 5-6 frames in it ? Possibly more.
I would go along with what the 'Committee' has said;

with regards using DIY style plywood. The problem here though is, that the inner material is often very soft - and it has a lot of voids/gaps/large spaces in it. You may find you would have difficulty using brass pins into this centre core - they tend to pull out easily.
I myself normally use a firm called https://www.mantuamodel.co.uk/
Mantua Models sell a birch faced ply which is very high quality; but, I am sad to say - it is very expensive as well - last time I bought some I paid £9 – plus postage and packing for a piece ⅛ inch thick x 1’ wide x 3’ long. As I say, it is expensive, but, well worth it for the quality of the material.
Aye
John e
Bluebird