RAW does not state anything about using hands, ex casting a spell, or attacking while moving in a glide.
[ +- ] Glide
Using your feathered arms, you can slow your fall, and glide short distances. When falling you can use your reaction to spread your arms, stiffen your wing feathers, and slow your descent. While doing so, you continue to fall gently at a speed of 60 feet per round, taking no fall damage when you land. If you would fall at least 10 feet in this way, you may fly up to your movement speed in one direction you choose, although you cannot choose to move upwards, landing in the space you finish your movement. You cannot glide while carrying heavy weapons or wielding a shield (though you may drop any held items as part of your reaction to spread your arms). You cannot glide while wearing heavy armor, or if you are encumbered.
Then Heavy glider states
[ +- ] Heavy Glider
Prerequisite: Glide trait
You were either born with great strength, or trained hard to allow yourself to glide under circumstances most would find impossible. You gain the following benefits:
You may glide while holding a heavy weapon and wearing heavy armor as long as you aren’t encumbered.
You may choose to land your glide in a space occupied by a hostile creature that is Large or smaller. If you do so, you may roll an opposed Strength check against the creature. On a success, you push them 10 feet away and knock them prone with the force of your impact. On a failure you land in the nearest un-occupied space.
So I should be able to "holding a heavy weapon" and/or hold a buckler and weapon, glide, then attack just fine.
The inability of not using a Glave while gliding makes sense as it does state you have to spread your arms
I was picturing the buckler more as a hand held only item. I imagine strapping anything to my arms/wings and trying to spread them might be hard/painful. Besides Gliding does say you can not do it with a strapped on shield.
The question is : Can I use a whip or other single handed weapon. Yes/No?