Your question is a very good one, and has been met with the sound of crickets for 36h while we all wait to see who has the answer! So it may be better posted to Chat with the eBay Community Team on Wednesday. In the meantime I've tried to find a temporary solution, as no Seller wants to pause sales or be out of pocket for the tariff payment on Custom International Postage.
The Post Office told me on the 28th that Royal Mail to the States requires tariff payment plus £1.50 RM admin fee at the time of posting. But that DPD and EVRI services purchased at the same counter do not. It seems couriers are allowed to leave tariffs uncollected at time of posting until February 2026. So a temporary measure may be to use such a courier (Buyer will be asked to pay before delivery, which is more normal).
Also PO staff asked: how United States Postal Service delivery partner in the States would know that the tariff was already paid? Her opinion was that USPS has insufficient access to the RM system (see similar posts about people being double charged VAT). So again, maybe best to let the Buyer pay all taxes at the time of delivery.
eBay recommends GSP which collects tariffs for the Buyer at time of purchase. I understand that you do not want to do this. You could do something similar to what Simple Deliver sellers are doing for combined orders: cancel and then relist with the tariff % + £1.50 added (with the Buyer's consent). Or if your main market is the States, you could spend a week adjusting your 1000s of item prices by hand, as you say 
The situation is still fluid, with tariffs found to be unlawful since your post, so personally I plan leave things be and take it one sale at a time, rather than spending hours adjusting something that may change again soon. Although I did add the £1.50 to all my USA international rate tables so I don't have to explain yet another charge to Buyers.
