If you your idea of moderate winds is 11 to 16knots i.e Force 4.
I would get the 145 Litre board, considering the description of the conditions, and your main goal is planning.
I also sail sometimes on an inland lake, at a club. I keep my own board 100L on the coast, which is a big board for its location. But when I sail inland and I want to get planning, AND I think the I wind is strong enough, which it hardly ever is...

I Will grab a 144 litre board (futura), and 8.5 metre sail. I weigh 70kg. Otherwise I grab a much smaller board and practise my swimming... and forget about planning.
A board of 145L and sailor of your size should be able to handle a sail up to 9m, before it gets too hard and expensive for where your at. I can and just started this year.
At the club I was referring to the futura/go 133L is the most popular for the conditions followed by 144.
If you trying to improve your planning ability in very marginal conditions a lighter board may help, but don't expect much improvement from your rio, if thats what your after. From a techinical perspetive there is probably a very good reason that there is a maximum width prescribed for formula boards, otherwise they probaby be even bigger than they are.
As someone said you need define what your idea of moderate winds are.