On apres diet days I`m your weight; post christmas; holidays; weekends I`m 3k heavier.
On 121 litres dont think you`ll gain much goiung over 8 metres; bigger sailors need more float /width to carry more sail.
I reckon youl`ll be planing in 12 knots + on your 121.
If you do the maths and include board weight;rig weight and wet suit/harness you dont have much reserve left. At 100k a 121 is really quite a small board; and will sink when wind drops.I struggle carrying 8 metre on 130 litres ;(but its do able)
Suspect you could do with 140 litres ish for medium winds and then 100 (ish) for high winds. ??? Forget light winds unless you fancy Formula !!! (IMO)