I ended up changing the slipping cogs for new ones rather than waiting for my new chain to wear out a bit and settle down.
I think the new-chain slip effect may be partly non-perfect gear-dérailleur alignment coupled with the way in which new chains, with less side to side bend, demand more accuracy.
My (made to bend, made to be unique and therefore expensive rip-off) dérailleur hanger is a bit bent but I don't want to buy another. With new gears the bike works.
Last edited by timtak; 05-27-22 at 06:11 PM.