I wanted to make weapon items for the various levels of Shadow Blade, e.g. Shadow Blade (Lvl. 2).

The reason is so it can show up in my action list and auto-calc hit and bonus damage.

I can make a weapon, change it to psychic damage, and add 2d8 damage. But whatever weapon I base the item on, I don't know how to turn off its base damage, e.g. 1d4 for a dagger which is already finesse, light, and thrown (20/60).

Can anyone help me figure out how to remove the 1d4 base damage?

I tried making it as a spell, which was nice because it would auto-scale, but I couldn't find a way for it to recognize proficiency bonus and max bonus from finesse and I'm trying to avoid making it a static manual entry. Making a battle master hexblade that will be using it at 4d8+DEX with three attacks with pact and hex weapon offhand.

You can’t do that yet. People have been requesting that they let us change base damage for quite a while. 

The closest you can come is to add “custom actions” directly on the character sheet.

Please add your request to this forum thread, the more requests the better!

I'm guessing they'd prefer to minimize cross postings, I'm sure a moderator will drift over this one anyway.

I've gone ahead and setup a custom action as it's the only current fix. If one of my players needs the same thing done, I have access to their sheets so I'll set it up for them manually. A feasible nuisance in my case, luckily.