There are a lot of breeches and hunt coats available now in light weight and stretchy fabrics. I compete in Hunters and over the past year I replaced my older hunt coat for a new RJ Classics soft shell hunt coat and an Essex Classics Talent yarn show shirt – totally love them both. I could ride in them all day and still be comfortable. There are a lot of Facebook groups where you could find some good, used (a little cheaper) show clothes or even Ebay. If you find new/used show clothes you could then sell your old ones to help with the cost. It looks like you ride/show hunters so you wouldn’t be able to show in full-seat breeches.
If your boots are slippery you can use a little sticky spray on them like Sporty Sit-Tite Spray. When you clean your boots make sure you aren’t cleaning the inside of your boots (the part that touches the horse). You want that part to get a little roughed up and a little ‘dirty’ so it gets grippy.
Good luck with showing and I hope you find something that works!