I have "a few" ideas.
Phantom membrane: slow falling (& night vision)
Rabbit's foot: jump boost & luck (luck is useless but why not I guess)
Heart of the sea: water breathing
Conduit: conduit power & glowing (it's a weaker version of the structure: conduit because 1.it doesn't damage drowneds and guardians 2. t doesn't omit light)
Turtle shell: resistance & slowness I or II
Wither rose: wither (cuz why not)
Nether star: regeneration, health boost & glowing (so it's not stronger than a beacon)
Dragon egg: saturation (it's a better version of regeneration)
Netherite block: fire protection & resistance II or III
Wither skull: wither, fire protection & strength II
Black wool: blindness (for multiplayer RP or something I suppose)
Totem of undying: here of the village
Pilliger flag: bad omen (or maybe any other flag instead, as a declaration of war against them)
Packed ice: speed
Glow ink sac: glowing (& blindness) (if you do do blindness maybe make normal ink sacs give blindness too)
Glow berry: glowing
Bucket of axolotl: dolphin's grace (& weakness) (because regeneration for this is a little op, plus it's cooler and makes more sense as a dolphin in your pocket, the weakness is because you have to be careful to protect axolotl when in combat, you wouldn't want to drop, hit, let it get hit or shake it too hard that it slams into its bucket)
End crystal: regeneration I or II
Enchanted golden apple: absorption (basically any level works, they're temp hearts that can be regained by re-equipping which balances out cuz it's risky to do during combat)
Bee hive: poison (cuz the natural one, the bees could just crawl out and sting you)
(Spyglass: night vision & slowness (the slower the effect, the more extra zoom you get))

I think unless it's a super difficult/rare item you shouldn't give it a lv2 or higher effect since that already is the strength of a maxed out beacon. In fact your shield and axolotl effects are basically game balance breaking levels better than beacons considering their cost and mobility.
Especially the shield, even giving resistance I seems op considering how low cost it is in addition to already being able to just completely block attacks from the front. Plus it's not like your going to put the shield elsewhere if you're going to equip it, meaning it's the same as having a mobile max beacon set to resistance with the additional ability of blocking attacks and explosions from the front. It should have no buffs or at least make t resistance I with slowness I or something like that to balance the game.

Of course the whole "good game balance" thing is just my opinion, if it doesn't matter as much to you, you can just ignore that part. Thanks for the cool pack regardless :)

I think you should make:
iron block= slowness and strength
feather= slow falling
cooked beef (Steak) = Regeneration
netherite sword = strength and regeneration?

I think that could be good on this datapack so i just posted this, Please don't take it seriously

what is this datapack never seen one like this.