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Full disclaimer : I acknowledge full well that in real life hybrid weapons are completely silly and impractical but I'm currently writing a fantastic-ish story where power fantasy is a key element. This hypothetical weapon will be more of a trick / makeshift weapon rather than a replacement o conventional polearm and bows.

Since there is a structural limit to how much weaponry a humanoid can successfully store on one's body without completely removing mobility,agility,speed, nimbleness. How would one design an effective or at the very least a decent polearm-bow hybrid? I was initially planning on having my heavily armed men carry a spear (2 parts, 1 hard-point to attach into a long ranged spear) + bow + quiver & arrows + throwing knives + a backup sword + smoke pellets or canister + boomerang + cable wires + 2 wrist mounted shields... But let's be brutally honest even with rule of cool factored-in that's just downright silly/ ridiculous and comically so. So now I'm trying to make compromises on rule of cool and semi-plausibility, which is why I'm trying to design/find shortcuts to keep my fictional soldiers as mobile/agile as possible without completely disregarding the fantasy element here.

Many thanks!!!

You don't. It's not merely impractical, it's straight-up impossible. A bow and a polearm require contradictory properties from their structural materials.

However, you can do something functionally quite similar. If you are not set specifically on having a bow, as opposed to merely some sort of high-power ranged weapon, then you can quite easily combine a polearm and a staff sling--basically, a small, human-powered trebuchet.

All that is required is a slight modification to the head of the polearm to include a sling-release hook at the end. The sling itself is basically just a bit of thin, knotted rope, which can be wrapped around the shaft when not in use, or just stuffed in a pocket--it is neither bulky nor heavy. And you always have the option of using the sling on its own if you like.

Suitable sling bullets are also considerably easier to source than decent arrows, which reduces the armory supplies you have to lug around with you, and eliminates the need to try to retrieve valuable arrows after a battle.

You change the bow to a crossbow.

The bayonet turned a firearm into a useful spear. You could start with a crossbow with a long shaft, like the ones in Central Africa...

... and just add a bayonet to obtain a spear.

Such strange hybrids are common in fantasy (I think D&D had them), but also in reality

I usually take "low-fantasy" to mean either No Magic or Very Little Magic, if you're going for a purely No Magic setting then Logan's answer is on the money, a bow and a polearm require the exact opposite qualities in the wood they're made of to function effectively. I your setting allows a little magic you may have a wood that has those diametrically opposed qualities. In this case the wood needs to be springy when pressure is applied across the grain, making a good bow, while remaining rigid when compressed parallel to the grain in a stabbing situation, thus also making a good spear, natural real-world wood doesn't do this but magic wood could. Or the spear could be enchanted to be bow-springy when the bowstring attached to it is pulled while being made of highly rigid wood that normally doesn't flex.

Yes, it can be done, but the level of craftsmanship required will make it impractical for mass production. It will have to be a specialists weapon, with the majority of each squad having conventional pole arms while one marksman carries this hybrid.

Also, it will be closer to the function of a crossbow rather than a conventional bow. Tactically, that means that from a distance the marksman can fire and then reload quickly for another shot, but once close combat is engaged, there will be no opportunities for reloading. In face-to-face melee, you will only get one shot.

The design of the bow portion of the weapon involves hollowing one end of the pole and turning it into a tube to hold the arrow. The bow's wings will project out perpendicularly from the tube, but will be mounted backwards such that they fold forward (towards the tip of the tube-enshrouded arrow) when the bow string is drawn. The bowstring itself will be drawn down into the tube down past the arrow's feathers and held by a hook which can be sprung crossbow style, by pulling a trigger mounted near the middle of the pole's length.

The arrows will be slightly different than conventional design, having feathers running all the way up to the tip. These feathers serve the additional function of holding the arrow centered within the tube which greatly increases the weapon's accuracy when fired.

When cocked and loaded, the bow wings of the pole arm bend down to meet the arrowhead. When in this position, the outer facing edge of these wings are sharpened such that the wings and arrowhead join together to form an over-sized spear point. Stabbing someone with this giant spear and then pulling the trigger would cause a sure-kill wound as the sharpened blade wings swung out in opposite directions, cutting through flesh with brutal force.