Thanks for your detailed reply and further explanation of your store and products. Sorry to hear that the process will be so cumbersome for you to create this override. As I mentioned, I have already submitted this feedback to our developers on your behalf.
To clarify, you have products that contain variants where not all variants need this specific tax override, is that correct? This is why you’ve stated that you need to create new products for each variant. Just would like to confirm this statement before confirming your posted next steps.
If yes, the reason you can’t add products that contain variants to this new manual collection for the tax override is not all variants require the override, then your listed steps are correct. As it’s not possible to only add specific variants to a collection, as you mentioned.
However, if this is not the case, you shouldn’t need to create new products for the variants if all variants will require the override. When you add a product with variants to the manual collection, and then add the tax override to the collection, all variants contained in that product will now have the assigned tax override.
Let me know if you need any further clarification here!