I set my flexwatt to 105F sometimes in the dead of winter but i also only use ambient room temp as the full cage temp. So while the whole cage is around 77-78F in the winter, i keep the flexwatt running a little warmer. Rest of the year when it's warmer, i turn the flexwatt down to around 100F which equates to anywhere from 87-90F as flexwatt varies from spot to spot, just the nature of the beast. Now for me, I'm looking at about a 10 degree difference between temp setting on the Herpstat and actual inside floor cage temps with my temp gun. So you will probably need to play with it.
And honestly, to me 108F seems pretty high. Usually AP cage variance is 5-10 degrees, so 108F would mean an inside floor temp of 98-102 which is fine assuming its the dead of winter and rest of cage is running cooler like 78F. But you are running a RHP too, you shouldnt need to run flexwatt that hot.
And i dont see how you got an 88F hot side with running flexwatt at 108F. How are you reading temps and from where? The top of the substrate or the inside cage floor? Should always read temps from the hottest spot which is the inside cage floor and set according to that as snakes will push substrate aside.
As for humidity, I have that same problem with cypress. Usually takes about a week for it completely level off. Funny thing for me is my BP actually hated cypress and was out nightly pushing and pacing. She likes aspen and any coco substrate though. So i switched her to Reptile Prime as i had a bag of it laying around and she likes it. All my boas, dumerils and retic love cypress though. My BP has to be the unique snowflake and go on fasts and be picky of her substrate lol.