New circuit in conduit: advice, please?

Hi,
I need to run an additional 20A circuit to the second floor from a finished (drywalled) basement - no good inside routes I can find without multiple drywall patches (and likely a lot of frustration). Here is what I would like to do:
NM-B (12-2) from service panel in basement to J box inside basement but near outside wall - connect to THWN in junction box - run THWN (12awg, three wires) to exterior and up the outside of house to enter attic. Junction box inside attic to transition from THWN back to NM-B 12-2. All THWN - from junction box in basement to Jbox in attic - would be inside 3/4" PVC 80.
Now for my many questions -
First and foremost: plan OK, or is this flawed/over complicated, or should I cut up drywall in many places and try to go inside?
What intervals to secure the PVC to the house - every 4ft?
Do I need an expansion fitting in the PVC (20-30ft vertical run outside ( Northeast US)
Can I run up to 6 12awg wires in 3/4 PVC 80 for up to two 20AMP circuits? I figure I might run addl wire while the work is being done in case a separate 15 or 20AMP circuit is ever needed upstairs.
What have I not considered?
btw, had two electricians come out and were given two very different approaches (UF cable under vinyl siding or PVC outside with a full run of NM-B 12-2) - neither of which seemed right based on my limited reading, so I'm trying to educate myself further.
Thanks!

I've seen it done a number of times like this with pipe run up the outside of the house. Personally I don't like it but if it is the only way then sometimes you have no choice. PVC tends to sag with outside use so I prefer to use emt. I don't have my reference chart in front of me but I think that 3/4 inch pvc schedule 80 can take up to 12 conductors of #12. Do you have an unfinished attic space above the second floor? You may be able to snake the wires with less damage to the drywall than you think. Is this a fairly new house with fire stops throughout? Sometimes you can drill through the top plate from the attic and through the bottom plate from the basement and get a snake all the way up. Look for an alternate route such as along a chimney chase or soil stack. Are there any closets that are one on top of the other or interior walls that do the same thing? Sometimes you can go through one of these walls or a closet and then into the attic space and snake down to where you need it.

Thanks Teamo,
house is 10-11yrs old. Agree I'd rather not do it outside. I spent many an hour staring at the walls and running up and downstairs to try to find a common inside wall, but the layout of the house + the finished basement severely limits options. Thanks to hole saws, openings around the vent stack are minimal. That said, it might be possible to go up one of the stair case walls, but would need to cut drywall in at least two places and not sure if there are fire stops along the way...

Inside is always better, but not always the easiest.
Forget about both elecrtircians.
One is a mistake waiting to happen with the uf under the siding, and the other is wrong about running nm-b in the conduit.
You could run uf from the panel, and inside the conduit to the attic.
This saves connection points.

If you think you may need more circuits for the furture, run 12/3 and then runn 2 hots, a neutral and a ground inside the conduit.
inside the panel, both circuits must be attached to a 2 pole breaker.

Thanks to all for the advice and confirming my concerns about the two who came out. Anyway, I spent the weekend running wire: I actually found a route inside: across basement ceiling where it drops down to accommodate the ductwork/ I-beam, through basement to inside corner of garage where it runs up the corner in a raceway to ceiling, then up and inside common wall from 2nd floor to attic, then across the attic down to laundry. Only had to cut one small piece of drywall in garage ceiling to get back into the interior wall. It was a pain to pull the wire (probably because I'm an amateur), but it's done. Thanks again!