I think maybe we should be only allowed to have one stack of each particular grenade, so 6 nades in total.
Think if you had 1 nade per slot people just wouldn't pick them up very often.
Blast radius is fine as it is I think
This might be an unpopular opinion, but I find the grenade system quite balanced actually...
They can only stack per 2, so carrying a lot of them requires a lot of backpack space. Space that you cannot use for meds or ammo. I often find myself questioning how many meds and ammo I probably need and balance that out against the grenades. The available backpack space is directly related to the number of grenades I will carry.
So instead of nerfing the grenades (which I find unnecessary) one could also reduce the number of blue/purple backpacks spawned.
Also right now many players seem to struggle countering grenade spam, but that may be because Gibraltar's dome shield is the perfect counter but isn't picked enough. Maybe if Gibby gets picked more often, the (IMO non-existing) grenade spam problem is by itself solved.
Another discussion would be the balance between the grenades. I find the arc star to be the most useful. The termite grenade seems to be very situational. If I had to choose what to do, I would probably increase the duration of the ground burning of the termite by 40% to make it valuble as disengage grenade with a potential of switching up the meta.
Range of all grenades feel good IMO. Unrelated but I think the effect range around the rockets of Bangalore's ultimate should be slightly increased. How did I end up talking about Bangalore...