For the breaker panel, I'd build a custom sized, elevated door and gasket all four edges.
Easily accessable, yet visually invisible. So that reflective panel, or the fabric panels of the Lemonade,
could be leveraged to neatly deal with that issue.
Could the bathroom door be moved forward, in front of the riser? Is it located where there needs to be a
surround speaker (in any of the locations)? So I'd keep a careful eye on where that door ideally ends up, in any case.
One advantage of a full width riser is you can work in a full range absorber. Not exactly HT 101 design stuff, but it
might be a factor to use this approach.
A smaller riser is simpler and cheaper, though...
If you were to use 3/4" plywood for the soffits, side and bottom, you could use a biscuit joiner to glue the soffit up, and
save another 3/4" of an inch width-wise. (Might be a little too OCD there...) You also might consider a few support drops,
between the HVAC sheet metal runs, to guarantee against a large plywood soffit sagging over time.