You have to avoid summer months Jun, July, August.
I think that September be optimum, warm but not hot, less wind, warm water.
In 2014 we went to Greece with SIB Honwave 3,8m air-floor and HONDA BF20.
Kids were smaller and we survive. Below picture from south Crete Lissos:
In all places in Greece waves were from the open Sea so they were significant.
We used below mentioned anchor (2x5kg)
But it was not enough and once we have been grounded:
Wave put (gently) boat and us on the middle of the beach ...
SIB was keeping on anchor and rope to stone on the beach, we swam around. A few big waves did come and one of it lift boat up (including anchor) and very gentle put boat on the beach. Lend lord had to go and bring us SIB wheels (starting from this moment I always used these wheels in Greece)
And longest trip was about 37 km and it was not enough for distances in Greece:
So we returned with RIB to the same places again and made some boat trips like 150 km and more.
With SIB and small RIB I would suggest to go to Croatia (Murter island).
Wind is mostly from continent and waves are mostly small near coast.
Please check my thread about Croatia. I like both (prefer Greece - much warmer Sea and air temp, cheaper food in restaurants) but easier conditions for boating are in Croatia