When we purchased our present house there was already a run of 3'
diameter gray PVC conduit run in a chase from the unfinished basement
to the attic above the second story. There are about 6 runs of
12-2/gnd Romex run from through this (as well as a few telephone, cable
TV and home network cables).

I am bothered by several things about this arrangement, and would like
to clean it up if practical. In part this is driven by a requirement
to run several new circuits to the attic to feed some new remodel work.
Also, quality / code wise the present Romex exits at the top of the
PVC conduit, is folded over the edge, and is banded to the conduit with
electrical tape. This bothers me, as I'd rather not use this approach
for new runs.

My concept involves:
1. Cutting the existing Romex where it exits the conduit in the attic
and pulling it back down to the basement.
2. Adding two large junction boxes, perhaps 12" x 12" x 8", one
connected to both the basement and attic ends of the 3" conduit.
3. Replacing the vertical runs with #12 THHN. This would use less
volume, and simplify adding additional circuits.

My questions are. Is this OK? Better that existing / Worse? Other
suggested approaches?

A big part of what bothers me about the existing arrangement is the
"hanging weight" of the Romex vertically unsupported through 2+
stories. Just doesn't seem wise. So several separate questions:
1. What is the allowed unsupported vertical run length for #12 THHN?.
2. Would I be aby better off, to keep the wires from 'swinging' (can't
think of a better word) inside of the 3" conduit, to run several 1" EMT
lengths up inside of the 3" and put the THHN inside of thise?

Thanks for your thoughts.
Bruce