as so much of this is going to be determined by access to the mbr suite wall & area,
lets talk about your pictures. if you'll give me a little more detail so
I know what we are looking at...it will help.
picture 1
this is the between floor area ...where?
are you taking these pics thru an attic access?

picture 2
this is un-insulated metal duct between first & second floor? serves what rooms?
picture 3

The third picture does show the duct to the master bath kick plate. This is at the end of the trunk.
this is insulated flex duct... is trunk line sheet metal and ducts flex?
or some other combination?
just trying to get a handle on how your duct system is laid out.
or is this maybe added later, earlier you had said
that ducts were enclosed in drywall with the exception of the mechanical room.
where is the mechanical room located?
I realize that you may not know, as the house isn't original to you. but
I write/think aloud sometimes.

what I'm really curious about in this picture is the batts.
depending upon which way the picture is turned..you have batts both on attic
floor and the walls of the bathroom?
it is the batts in the bathroom walls that I'm trying
to determine if you have access to. do you have access or can you make access
to these areas?

picture 4
not sure what this is at all other than an insulation batt. but just looking
at the dirty ness of this batt..there is some type of air flow through the batt.
as the air moves through insulation, the dirt/dust collects on the insulation.
dirty insulation is an indication of air leakage. doesn't matter if
the insulation is on the floor, ceiling, wall or on the duct insulation at its
take off. if it is dirty, move insulation..there is the leak. big leaks cause
larger areas of dirt, small strong leaks cause more dirt to be collected in/on/thru
insulation.

picture 5
recessed lights...the bane of my existence...LOL. hard to tell...but if they have
holes in the housing they are IC (insulation contact), if no holes they are
ICAT (insulation contact air tight). with both, insulation can be in contact
with the housing. but if IC the insulation over time will show the air leaks
through the housing. years ago, we were taught in our training that one
IC recessed can equals one sq ft of un-insulated attic. because as air moves
through the insulation it reduces the performance of the insulation.
can't remember off hand what the infiltration rate of each IC light is..
but it is enough to make it worthwhile to retrofit IC to ICAT. there
are inserts you can install from inside the house to do this.

________________________________________________
now more questions:
can you see the housing of any of the recessed lights to determine IC or ICAT?

if not, you can take the bulb out from inside the house, and read the label inside the can light.
but...read the fine print. here, Juno recessed lights are common.
but the red & white sticker inside an IC light says in big print..
ICAT..then in small print...when used with the following inserts/trims.
the true ICAT has an orange & white sticker. you have to read the fine print.
it is very misleading otherwise. this info is for Halo/Juno & may not apply to
other brands way of defining IC vs ICAT.
remember that each of these cans have a hole in the sheetrock that
is covered by the trim. so it isn't only the can itself, but the penetration
in the drywall that leaks attic air into the house.

at this time, there is no insulation between floors of first and second story?
how much of the second floor has walls shared with attic space?
what type of beams are used for second story floors?
open web trusses? solid trusses? 2x's?

sorry for the long post..just hard to tell with the pics & need the info
to determine what can be done.
I'll check with mods if we start to venture into diy area. if so you can email me
by clicking my user name and going to about me tab.
hopefully it won't be necessary...but just in case.