If you are hung up on something that basic,maybe you should rethink your desision on acting as GC for the remodel. I would never penitrate a roof to install a fart fan. If the atic is vented as it should be I don't believe venting a bath into it should present a problem.

Your asking about HVAC in your heading and then an exhaust fan in the body of your question? klem1 is correct.......

Don't vent your exhaust fan into the attic. You can introduce moisture into the attic space that could lead to mold. My understanding is that this is also not up to code in many areas. I would prefer to vent out of the side of the building than through the roof unless the roof was due for replacement. If you don't do a great job sealing the opening in the roof, you can introduce a leak point. Having a longer run for the exhaust fan may reduce the efficiency but not by very much depending on your model.

Regarding your HVAC question, I would move the vent once demo is done. Will you be hiring someone to do this? The same person should be able to install your vent fan. Use an insulated duct for the fan. Look at fantech for parts and research the forum for some best practices.

Have you selected a vent fan? Panasonic and fantech fans are well liked on the forum. Remember to plan for a timer switch for the vent fan.

In my humble opinion, whether you hire a gc or not, you should have a clear understanding of what you are paying for and the order of work.

Good luck!

Can you run the exhaust duct vertical up into the attic, then horizontally out the side gable wall?

With a long run, you can install a remote inline fan in the attic itself. The bathroom will simply have the ventilation grate in the ceiling. Check out Fantech fans.

Do not vent the fan directly into the attic space, even if the attic is open to free air. Bad, bad, bad.

As far as the HVAC portion, it can depend. Plumbing is rigid and waste pipes have to be set to a specific pitch for drainage. I'm not a big fan of flex HVAC, but there are times when its use makes sense.

So prioritize. 'Larger" and "more rigid" and things that need to be set at a proper pitch (waste pipes, for example) get to be set first. "Smaller" and "more flexible" get set last.

Sometimes the depth or width of your joist bays will allow them to coexist in the same space.

I think you know what has to be done and you're simply asking for confirmation. You're on the right path.

Draw out your floor framing with joists and joist bay spaces. Superimpose the location of your plumbing fixtures over that. Then draw in your rigid waste lines which have to be pitched for drainage. Then your HVAC as needed if it's rigid duct. You have more flexibility with flexible duct (sorry). Plumbing supply is small diameter and can normally be routed around things. Copper is rigid, PEX can waffle here and there but needs more support. Soft electrical can go anywhere and gets what space is left over.

Thank you for the help (and you know who you are...)!!
I hadn't heard of Fantech. In our other bathroom we have what I thought was the standard-strength exhaust fan but I'm not too impressed, so I appreciate learning about better options. Just trying to think outside the box!
Moxie