Hi there, some customers found the board nervous and prone to spinout, when using a lot of back foot pressure/short harness lines/high boom or too big sails. By standing more upright, lengthening the harness lines and applying less back foot pressure with reasonable size sails, you should be able to minimise the problem further.
You can choose a bigger centre fin or ride it as a single fin too, there are multiple options on fins which you can read about here on the various threads too - but generally a FreeWave style fin between 22-24 for a centre fin will work well. Single Fin like the one we supply with our standard freewave model, in 28-30cm should work? hope that helps, along with the trimming tips, we sold a few thousand of those boards and the majority of the customers were really happy with the STB.