Hi Christos,
welcome back to the forum, have not seen much from you lately! Yes, the 76 is a great board, my go to model for powered 5.0 and smaller conditions, if the 82 is feeling a bit big/bigger waves & choppy. But it does reach it´s limit in lighter winds of course.
4.5 is a touch on the small side for the FW85, I´ve tested it with a 4.7 Super Hero quite a few times in the waves, in Cape Town and Hawaii, during R&D - feels great on even landed a 360 with it, so it has plenty of performance. My wife uses it with a 5.3 at Lake Garda, so it has a massive range of use.
I would think about getting a 4.7 or 5.0 to expand your range of use with the FW, as a bit more power might help on the much lighter days, even if you are fairly light yourself. It´s always hard to define an exact wind range as it does depend on rider ability, pumping technique etc. I´d hazard a guess and say you could plane with 15 knots with the 4.5 if you have good technique, but normally you would need a 4.7/5.0 for that amount of wind. I´d say actually from 12-15 knots with a 5.0 you should be fine.
Generally my experience is that a lot of the 5.2/5.3 sails, do not really plane much earlier than 5.0´s, except in the very lighter winds, when lightweight riders are using them (heavier riders for sure benefit from the 0.2/0.3 extra sail size. But often a 5.3 paired with a 80L board can tend to push it down into the water, so you plane about the same but the bigger sail feels much heavier/only really helps to keep planing.
On the other hand, my favourite two sails on the FW 85 are 5.0/5.3, but I´m 78kg´s and like to sail quite powered up/jump? So, to summarise - yes, I´d take the 85 even if you do not invest in a bigger sail (but would reccomend it, also test before you buy). No, I don´t think the FW75 is the answer as it will only be slightly better than your Grip 76 in planing. Hope that helps, good luck!