I would get something in the 9'0" to 9'4" range. There are SO many more longboards being built in that length, giving you way more options.
Second, if you decide to sell the board, most people looking to buy a log are going to want something 9 foot or longer. Well maintained longboards are easy to sell for decent money after many years of use, used 8'6"s not so much.
Amen! Surfed a 9'2" NSP in double overhead surf in Guiones . . . Getting out was . . . Getting worked ! My turtle rolled so much it was waaaaay dizzy. But I could catch and get down the line, cut back . . . And that isn't a true hplb.
Make sure it's not too long to fit in your vehicle so you can lock it up while you chow down after a go out. Go with some nose rocker so it can take a decent drop, and some tail rocker so it will turn. Otherwise it's just a log to noseride on knee to waist high grovel. A finbox plus two removable side fins set up (Future fins are faster than FCS) will give you lots of options for different sizes and conditions. A small battery powered trolling motor can help you paddle out during rough days.