Such a creature probably wouldn't look much like anything based on Earth biology. But given that it's genetically engineered, perhaps this can be contrived.
It needs to be fast moving. Perhaps not as fast as a jet or even a humvee, but able to move at human sprinting speeds, let's call that 15mph+. It needs to be difficult to target, so smaller than the typical infantry soldier. Anything smaller than a cantaloupe should suffice. But perhaps even smaller is viable... something the size of a grasshopper isn't entirely ruled out, provided that it can offer some threat as you've outlined.
To offer threats to infantry, this thing's just going to have a stinger with some vile form of venom
For tanks, it's more difficult to think of how it could disable the tank (but let's face it, even when the crews are murdered, sometimes those are just hosed out and welded together again). So, the primary mode of attack will be to attack the crews, which are rather vulnerable even if it seems like they aren't. Tanks need air, they aren't self-contained atmospherically.
If we have the insect-sized attackers, you'll want a swarm behavior to clog air intakes, both to stall out the motors and to choke out the crews until they attempt escape.
But if you could have the melon-sized attacker, I also wonder why it couldn't be programmed to stuff itself down the barrel. It wouldn't be foreworthy until the obstruction was cleared, without that firing the gun risks blowing the thing. Such a tactic could put the thing out of commission for days. And if the organism were capable of excretions that could permanently foul the barrel... just wow. For instance, there are some deep-sea organisms that can secret metallic iron, depositing it in a way reminiscent of electroplating. I'm not sure how much would need to be deposited on the inside of an M1 barrel to make it permanently unfireable, but it's not some massive amount either, not a quarter-inch or anything like that.
Such an organism would ideally be "protoplasmic" in its body composition, with no distinct organs that could be damaged (immune to small arms fire). Unable to fly, they might hide away in dense foliage, waiting for a tank to come through. Even up in trees, waiting to fall down on top when they sense tanks rumbling by underneath.
The insect model is nice too, because with flight they could be quick and deadly. Swarming would be horrific, and large swarms might even be resistant to flamethrowers (in the "we can weather attrition more than you can" sense of the word).
Your super-murder-hornets might, for instance, have some novel venom that is simply corrosive in addition to being ordinarily venomous... there won't be much antidote if it's eating away the flesh that it was injected to. Or maybe they just harbor the microbes that cause necrotizing fasciitis. Thus even if given the antidote, they will only recover from the venom and not the resulting infection. Their stingers aren't one-shots like honey bees, they can run around being vicious little shits. Long enough to penetrate most chemical warfare protective equipment, and all of the uniform save maybe the boot leather.