Not SB team here, but pretty easy question to answer
It does sound like a formula could be the wrong board for you.
Your pretty light, and although on paper, your K105 should be a decent light weights summer board, its not. The rocker line/bottom shape is control biased, meaning its not designed as a light wind early planner.
Don't fall into the volume trap, look at board tail and max width for indications on its intended design range. A modern slalom board will have a massive "low end" advantage c/w a cross-over board like the Kombat. IE, an iS86 will get going sooner than a Kombat 105. 20 litres difference between both these boards, but they both have the same sail range, 5m to 7m. This shows how more efficient the iS is c/w the kombat.
A 3rd generation formula board will not work at its best if you only intend using an 8.5 to 9m sail. They need much more power/lift to ride correctly. So this is not a good option for you, unless you intend using a 10m/11m+ sail.
For the sail size you mention, 8.5m to 9m, you will way better going with either an iS121 or iS131. If your sailing in NO LESS than 15k winds, the iS121 will be perfect. If your sailing in flukey/gusty winds or the wind MIGHT drop below 12/15k, then you should be looking at the iS131.
Its also worth remembering that if you sail an iSonic in its lower wind range, whilst not to technical, they require an active sailing style. Meaning, you can't just sheet in, and away you go
They need more imput.
Sorry to waffle-on, but like most questions, there's no simple answer. Theres always more than one way to cook a goose