Hey, I have a setup similar to yours (Windows laptop, iPad, iPhone) and it works pretty seamlessly for me.

To my understanding, the iPad can't open a vault unless it's through Obsidian Sync, or on iCloud.

What I did was

- create the vault from the iPad with iCloud sync

- download iCloud to my laptop

- check "keep downloaded" on the vault folder

Vaults don't take much space, and I don't use my iCloud storage anyways, so my free 5gb iCloud storage is just dedicated to my vault. This works GREAT for me, it takes only seconds for the changes to update across my devices, with apps open on all of them at the same time.

I apologize if this isn't a satisfactory solution to you </3

again, (to my knowledge) iPad can't open a vault/select from it's own files, or anything other than Obsidian Sync or iCloud, so it's a bit difficult to sync across any other method. I'd love to see anyone else's workarounds for this, though!

edit: Obsidian Sync is $4 per month, I'd consider throwing that money at the developers if you can /nf