I getting ready to reroof my house, to standing seam metal.
I have two toilets, 4 sinks, 2 showers, one washer.
I have an accessible attic.
Can I plumb all the current vents that pierce the current roof, into one single larger vent?

Purpose being to clean up roof clutter, move the main vent to back yard side of the roof, and reduce potential future leak sources?

Cost is not an object, but not sure if they will interact adversely, or long runs are a problem, or something else?

The short answer is probably yes this is allowed, but check your local codes.

The final vent size will then be driven by your sewer main size. Again check your local codes.

Also be sure the vent laterals are sloped correctly back to the house so condensation does not pool.

My house was built in 1983 and has only 1 DWV pipe exiting the roof. All the vent pipes from 3 baths and 1 kitchen are 2 inch and are tee'd into 1 3 inch stack vent. But I live 6 miles outside the city and county builder codes may have been different back then.

YMMV.

I can't help but envision the collectors on a set of custom headers coming out of your roof.

Google Studor Vent could help if you are going to have long runs to the main stack vent.

Not really an issue.....if you do not live in a freezing climate. The stack will freeze shut if you do and it gets real cold, most cold weather states required the stack to increase to the next size thru the roof for that reason. The new shacker houses with plastic pipe get thru inspections with a 3" stack, but it is wise to up it to 4 in a cold climate.

There are maximum runs to the common stack as well, and slope requirements to consider

And flow units, and vent fittings vs drain fittings, but a 4" stack is 160units, vs a 3" at 20 I think, so a 4" increased to 6"should handle anything he has, 1/4" per foot slope on the pipe.