In a similar way, I am playing and GMing RQ since close to 40 years: I have always played in Glorantha, but before RQG, I had never GMed in Glorantha. I have used RQ3 for 1 home campaign in Rome, 1 Vikings campaign, 2 Land of Ninja campaign, 1 SF campaign (with some psi powers built along the rules for lunar magic).
Frankly, if missing, RQG would have been completely different, and much less interesting for me.
My own tastes would have been to start from RQ3, not RQ2.