Change your vet.
This last thing that we have to do for our friends is hard enough, without the vet being unsupportive.
I think I am right in saying that human medics swear an oath to preserve life, which makes it very hard for them to assist someone to slip away. Vets tell me that they are so very glad that they do not have to swear that same oath, but rather to prevent suffering, so that makes these decisions easier and enables them to end an animal's suffering.
It sounds as though your vet has difficulty recognising when an animal is suffering and quality of life has gone.
As to your decision - and it is
your decision, not the vet's - about Freddie. It sounds as though you know it's time. Frankly, finding my dog stuck on his back when I return home, not knowing how long he's been like that, would be something I couldn't bear to see three times. You need support to tell your vet what needs to happen.
Personally I would ask to see a different vet who will not make this more difficult than it is going to be in any case. If I lived near to you I would offer to come with you for moral support.
Looking back over my life and dogs and cats, I do sometimes wonder whether I made this or that decision a little too swiftly and should have given this dog or that cat another few weeks, another op, whatever. But on the whole I feel it is better to not be looking back at prolonging suffering than to be wondering whether any particular animal could have had another few good days after a 17 year innings.
And one that I do recall was a wonderful cat, who at 16 had a leg amputated with what turned out to be an aggressive tumour at the site of an old injury. He never really recovered from the op but could still purr and want to be with me - but was off his food, barely able to walk, not anywhere near wanting to go out and about, and nothing like his former active larger-than-life self. For weeks I struggled with balancing giving him enough time to recover from the op with worrying about how hard he was clearly finding it all. One day I was sure I couldn't watch him go on like this any longer, and we'd had the histology by then so we knew the tumour was aggressive and therefore secondaries were highly likely. I took him in, and to this day I am positive he knew exactly what was happening and thanked me for giving him peace.
