Congratulations on your 356! Thinwater and shemandr hit a lot of the main points. On the B&R, you have to set the shrouds first and also have the sails trimmed properly to do an in-place evaluation.
Assuming the diamonds/cap shrouds are set up correctly, you should have about a 2" prebend with in-mast. Measure this by taking your main halyard or topping lift and holding it snug at the gooseneck; the max gap between it and the mast shows your prebend. too much will cause furling issues and a baggy main. the stock UK sail is pretty soft, and needs to be trimmed properly for best performance. In calm air, with the sail fully out, tighten the halyard until all of those luff wrinkles disappear. In 13-18 you should be reefed, at least the main - reef the main first. I reef the main and fly the full jib unless it's really blowing. the foot of the main should be tight to the boom and the sheet trimmed for proper flow. this will move the max draft further forward where it should be. Then use your traveler to control the main, easing it to reduce power and heel in puffs, and bringing it in to power up.Some forestay sag is normal, as is some bouncing - the mast is a pretty long lever arm and blasting thru waves can cause this. Once the mast and main are set, sail her - too much weather helm would indicate too much rake. the step by step tuning process guide is on the Selden website;it's hard to be definitive without hands on. She's a great boat, you 'll love sailing her! We've still got 15" of snow, more coming Sunday (sob!). Thanks for the video!