Came across this company a few days ago when I was looking for something similar albeit just out of curiousity as I know the previous mentioned Northwest repairs are US based,
https://www.bhawkgaming.co.uk/ there are a few out there in the UK who are able to do PCB level repair, reflow/reballs, SMD replacements etc, some of them do list their initial diagnostic and return shipping fee so you know as a minimum what you are going to be paying and depending on the outcome they also list repair and parts costs (GDDDR5/6/6x 1GB/2GB etc SMD capacitors, rails etc) the company I linked has a reasonable £60 diagnostic and return shipping cost, so that's the bare minimum to get a diagnostic and have it returned to you, obivously they would then advise the cost of any parts/repairs to bring it back to a working state, definitely not soemthing you would do with a £200 GPU though for higher end ones I can see the value instead of having to shell out and buy a replacement, assuming you have no warranty on the 6900XT or else you would be sending it in for RMA