You should be using Bonded Poly for outdoor use. Thread size? Are you going for a decrative contrasting color, if so 210-270. If not then 92 at a minum but 120 is the go to. Tanara is great thread, but finding colors other than black and white is a pain, and most of it out there is 90, and well over 100 bucks for an 8 oz spool. I've switched from COATS and Ultra Dee to Aman thread, easy to find colors, machine likes it better and the size options are great.
https://relicate.com/products/serabond- ... -30-tex-90