John, I am glad to hear that your gunsafe contents are safe (no pun) from moisture/corrosion issues while enduring such high RH periods.
I was satisfied with my "standard" setup of GR-type heaters only and unsealed gunsafe doors ... until I was not.
During that wet summer when I only had GR-type heaters deployed (my setup for several years by that time) I did find the beginnings of rust on two of my rifles ... the in-safe hygrometers were displaying RHs in the upper 60s (basement RH low 70s). My first attempt at corrective measures was to buy the desiccant units. They knocked down the RH, some, but maxed out fast, needing recharge every few days, IIRC ... so I bought a 2nd set so that one could be recharging.
Then I stopped and turned up the gain on my brain and thought thru the issue. I realized that the additional moisture-laden air was being drawn in thru the unsealed doors (via the convection current created by the GRs).
So, with a roll of foam strip that I had on-hand, I sealed one of the doors and continued monitoring the environment in both gunsafes.
Within a few days, the sealed gunsafe RH was down to 50% and the speed of the dessicant loading was reduced by more than half. Due to this I sealed the door on my other gunsafe.
I also added a constantly-running small fan in each ... provides constant airflow & adds a bit of heat.
Since making these mods my gunsafe interiors never get over 55%RH and the temperature is always a couple of degrees above that of the air in the basement.
When I see the RH crossing the 50% threshold (sometime after I have to start deploying my portable dehumdifier in the basement), I start deploying the dessicant units until such time as the RH in the basement stays in the low 50s with no help.
With my setup, I can fire chlorate primered ammo in a rifle/pistol, put it in one of my gunsafes and clean it at a later time because I keep the RH low.
I hope that your annual range trip goes/went well!
Kim