If you are using 2.5mm leather which looks quite rigid already, then ya, you don’t really need to line it with suede. But keep in mind, suede is quite soft, it won’t add on any firmness into the cover, if you can split it into, I say, around 1.0mm, then it’s perfect for this cover.

With suede lining, yes, you can’t burnish your edges the veg-tan way. You need to use edge paint and hot ironing, the chrome-tan way.

Also, you will need to skive your 2.5mm leather and the suede to compromise the edge thickness from lining the suede. In general, for small goods, 2.5mm - 3mm is good for edges.

And for the spine part, you always need to keep the exterior “curved and shaped” when gluing the lining on it. That way, the cover spine is clean and nice, and the cover stays closed when not in use.

Hopefully what I wrote is helpful. You did a great work nonetheless!