Engagement ring valuation versus cost disparity

Hi all, I have predicament regarding an engagement ring and wanted some input from more experienced jewellery aficionados.

I wanted to design a one of a kind engagement ring to propose to my girlfriend with. I did a bunch of research but could not find an affordable jeweller in London (where I live). I decided to head online armed with (i think) a decent knowledge of gemstone and ring values. I settled on a designer I found through etsy who had an impressive portfolio and excellent reviews. I verified her work through looking at her profile on various social media platforms in addition to Etsy. Together we designed a ring to my exacting specifications (taking quite a few months) and she helped me select a beautiful blue green sapphire approximately 2.5 carats in size. She also gives me its country of origin, name of the gem cutter etc. In total she was charging me £5100 for the stone and the custom white gold/palladium setting/band. I was very happy with it when I received it in May (looks exactly like the photos she sent me etc) and believed I paid a fair price.

Fast forward to this week when I finally planned on when to propose (delayed by COVID of course) and I took the ring to get a valuation for insurance purposes. The valuer is accredited and has GIA credentials etc etc. He looks the ring over, checks it with some sort of machine then sits me down and tells me that he values the ring at LESS THAN HALF of what I paid. I was utterly gobsmacked. He confirms that the gem is indeed a sapphire but sapphires are worth about £250 per carat making this stone worth between £700-£1000 and the ring in total worth about £1500-£2000. He is distraught on my behalf (which in hindsight seems a bit odd) he brings the ring to a gemologist in his office to verify his valuation of the sapphire (they agree with him) and he proceeds to take me to a local gemstone store (he is based in Hatton Gardens which has loads of them) and shows my ring to a sapphire dealer who also says its worth £250 per carat. He then tells me to raise a claim with my credit card and Etsy to try and get my money back. He goes on to say that I could get a very similar ring to what I have in Hatton Garden for £2000 (I'm not entirely sure I believe that given the unique design of this ring)

I find this mind boggling. I've never seen 2.5 carat sapphires cost anything less than £1300 (and thats for really dull looking ones) the one I have has a beautiful colours that ranges from clear sparkling sea green to forest green with hints of blue. I contact the seller and she is very surprised and upset (she has engaged with me on this very constructively and I just don't believe she is out to rip me off based on her responses). She broke down her costs on the ring for me: $2850 for the actual cost of the gem from a dealer, $1850 for the materials and labour to make the ring itself and $1800 which is her profit margin on the gemstone (total $6500 which is about £5100) She told me that most jewellers have profit margin of 2-3 times the cost of the gem and that her profit margin is less than this (any jewellers on here able to verify this?). She also gave me more details on the gem from the dealer on its clarity and grading which makes it a very good quality sapphire (and looking at it I don't doubt it though I am layman)

My predicament is what to do next? I really love this ring and I don't really want another one but I would really like to know if I'm being ripped off or not. Also what value should I use to insure this??

Thanks for listening to my rambling tale but I really need some advice!