I would consider myself beginner - intermediate and I want to know any tips or gear for getting better. I'm a light sailor (60kg) and I run a 170L starboard go with a 5.3m free ride sail. I can plane at 12-18 knots (board planes easy as), and I prefer planing without foot straps due to the board layout. I don't have a harness, so how necessary actually is a harness? My hands get absolutely demolished since my skin hates water and the boom essentially tries to de-glove me after get back after falling in. Would a harness help with this? I was also wondering about board and sail size. Would it be good for me to move to a larger sail and small board? Like up 6-6.5m range and a 120L board. Plus are smaller boards better if I try move to chop/swell? Any help would be greatly appreciated

Using a harness seem like the logical next step, that will enable you to sail for longer with the sail that you have and also open up for using larger sails. With the harness you will also want to start using foot-straps, in order to get a proper stance, i.e. light on the feet and transferring weight into the harness.

The Go has a bunch of footstrap options, from the way inboard "beginner" position, one in the middle, and one out on the rails. Once you're using the harness, try out each of them! I started getting comfortable planing with a harness with the footstraps in the middle position as it seemed like a good compromise compared to where I'd put my feet normally, but I'm also using a much bigger sail due to my heavier weight (75kg/193cm). After getting comfortable, I could tell that the straps really needed to be on the outboard position when properly powered up, so that's where they are now.

I am 67kgs, a few years experience now and still find the big boards good fun on flat-water.
I do like the feeling of sailing without using harness on big boards but it is definitely worth learning to use. Hooked in and in straps is nice.
It is like talking on the phone hands free, while flying.

This one is JP SLW 165ltr with Ezzy 8.5 sail
I also like RSone long board hooked in.