Metal selection for old Boston Mansard

Hello,
We will need to replace our old asphalt architectural shingle roof soon. We'd like. to have something more durable with less probability of needing repair. I'd really like a product that lives up to its warranty, rather than the unhelpful '30-year' claim on the asphalt shingles!

We were told by our current roof's installer at the time, that the architectural shingles would need to come off before installing new singles. I suspect this would be a good idea even with metal roofing which in some cases can be put directly over an older asphalt one. We need to make sure that the underlying wooden structure is sound, and that if it does need repair, that we would see it and do it before installing the new roof.

We live just north of Boston.
Our neighborhood has many houses 1880s, old Mansard style, as is ours. Most roofs here look to be some version of variegated asphalt shingle, either flat or architectural.

Our house is about 1400 square feet. It has:
1) two relatively shallow small porch roofs;
2) a near vertical section slanting inward from the top of the first floor, up past the second floor, to where it joins-->
3) the final relatively shallow 4-sided pyramid-shape.

The near vertical section and the shallow pyramid top are typical for Mansards in our area.

The closest ocean beach is about 3.5 miles. We do have a medium closer tidal river.

What I have read suggests that Al, Galvalume, and galvanized steel as most likely choices for our situation. For having some variegation in color and texture to blend in with our neighbors, we are tending towards non-stoned metal surfaces with 'slate' or 'shake' texture. We like the appearance of stone-coated products, but I read one installer's comments that suggested they might be more awkward to use, or to install properly.

Obviously there are a number of large-scale manufacturers, but also I see that other installers form their own 'shingles' and offer what seem to be equivalent warranties.

We will be talking with several installers into the coming winter.

Any suggestions, comments, links to other discussion sites, etc. would be welcome.

Thanks in advance.