Welcome to THR and to our wonderful hobby of reloading that is a passion for some of us.
Shop plans look great.
Some suggestions.
When my BIL asked me to build him an insulated 12'x24' room inside his 10,000 sq. ft. metal shop with plan to use 10,000 BTU window AC where shop got to 100F+ in the summer, I chose to use 2" foil backed polystyrene sheets against the west facing wall joined with foil tape and further insulated with R30 fiberglass insulation covered with sheet rock. Ceiling was lowered to 8' and R30 fiberglass insulation was used without any polystyrene sheets with sheet rock. Inside walls were insulated with R30 fiberglass insulation with 1/2" polystyrene sheets under the sheet rock. Exterior insulated metal door was used for R value and security.
Even though I framed an opening for a window AC unit, room remained in the 60s to 70s most of the year and upper 70s when shop temperature reached 100F. BIL said hottest room ever got was low 80s when shop temperature hit 110F+ and never needed to install the window AC.
So consider dropping the ceiling to 8' and increasing insulation to better maintain ambient temperature and lighten the workload for AC/dehumidifier.
I live at the coast where it is 100% humidity during winter rainy season and I reload indoors. Everything rusts here, even stainless steel gun parts and I found WD-40 Specialist Long-Term (Rated/tested up to a year in salt water condition) to be most effective in rust prevention. So now, all of my gun parts and tools get sprayed with WD-40 Specialist (Make sure it is "Long-Term" rated for 1 year against rusting as there are more than one version) then lubed with oil/grease and I have not experienced rusting issues. Over 45 lubricants were tested in this comparison for rusting under salt water conditions and WD-40 Specialist came out on top -
https://dayattherange.com/?page_id=3667
As to powder/primer storage, I store both indoors and return powder back to container as soon as I am done reloading. Primers are stored inside plastic bags that are taped. Just with these practices, I have not experienced any issues with humidity in regards to powder/primer.
To reduce lead intake from reloading activities, I sort/dry tumble brass outdoors and deprime where spent primers are captured through a plastic tube into a drinking water bottle -
https://www.thehighroad.org/index.php?threads/reloading-safety-exposure.896104/#post-12080442
Have you considered keeping bench height for standing but building a rolling bench for sit down reloading?
While I started reloading with standing height bench in the garage, I built 2'x3' portable bench with casters from Harbor Freight furniture dolly to roll anywhere in and around the house. 11 layer plywood was used reinforced with 2x4s for "no flex" bench top and with presses mounted at the ends, I could resize thicker military .308 brass even with bench empty on hardwood floor. If you load a lot, having a sit down option will be welcomed by your tired back/feet.