You are certainly doing everything you can as an owner.
The combination of dental issues and stones is extremely difficult to manage and there are at some point limits, financial, medical and care-wise that you will reach. It can be that one of the stones is actually more cigar-shaped and just showing up differently. I've also had a stone visible on an x-ray but when the booked operation came around, the bleep or stone had disappeared so Terfel returned home with a shaved but intact belly...

However, how to go forward and the pros and cons re. operating or not in view of his dental issues, age and fitness etc. is something you need to discuss with your vet. You could ask your clinic whether you can book a (paid) consultation over the phone with your vet to have the time to discuss it so you have the necessary medical background information on which to base any decisions.
Please be aware that the UK is mainly a hard water country whereas the USA and Canada are mainly soft water countries, so our calcium and diet recommendations may be a bit on the low side for you, which can also lead to calcium pees and occasionally stones. It is about finding the perfect balance for yourself in your personal location. The individual variables from water intake, pellet brands, available veg and hay vary. Any changes will take weeks to filter through the body and become fully effective. However, the stone formation process itself can be disturbed and there is not much we can do about that.
As to vitamin C: if you supplement, best do it as 2 week long booster courses so the body doesn't accustom to the higher levels and ignores them as its new normal.
This doesn't matter so much anymore in terminal cases as long term scurvy symptom slump concerns are no longer relevant.
You are not quite there yet but you are coming up towards the line where you may have to make some tough decisions, and - as you love and care deeply - of course you worry; and very rightly so. It is a daunting emotional maze that you are about to enter, after all.
Usually, you feel bad and guilty in yourself, just for being in that position. We humans are wired to seek any peceived 'fault' within ourselves although there is no reason for you to do so - you are after all trying your best. You are not expected to get everything right, unless you put yourself into that particular corner. Your own life, children and pets are an ongoing learning process where learning from mistakes is an important part of.
Of course you don't want to loose any piggies of yours in the first place but at the same time you also want to get things absolutely right for them; these are very conflicting emotions that happen all at once and can make you feel overwhelmed, disorientated or even paralysed. .
You have to be aware that your grieving process doesn't start with the death; it starts with the moment you realise that a beloved one - pet, human or even yourself - may not make it and that their time is limited. This is what you experience now and why you feel you need somebody what to do. The onset is always one of the very worst times.
There is no right or wrong within an area where it is very obviously too soon and equally obviously too late. Practicalities play as much of a role as your very personal feelings and the nature of the individual bond. If you listen to your heart of hearts and go with that, this will also take you to the centre of the maze and out again.
It is never getting any less painful although over time you know where you'd draw your personal line so you do not have to go through the whole process all over again but you still have to judge each case on its own merits. Generally you will know the day your piggy has lost their zest for life and has had enough. Whether to operate or not in a make or break setting is another scenario that can seriously throw you.
We cannot make those decisions for you - that is both the privilege and at the same the burden of ownership - but what we can do is give you the instruments at hand so you can work out for yourself where you stand and where you draw your lines for yourself and for that specific piggy/pet.
This will then help you make your own decisions with a clearer mind and, as a consequence, you'll hopefully suffer less from self-doubts and feelings of failure and guilt in the longer term. The pain is unfortunately never any less because every bond is unique but we can help you make the best of any remaining time.
I never like linking in these two links of course but you may find that they address lots of aspects currently going through your head or that you cannot express yet. I have tried my best to keep them as practical as possible and to avoid glossing over the sticky points where you need a steadying hand the most.
A Practical and Sensitive Guide to Dying, Terminal Illness and Euthanasia in Guinea Pigs
Operation or Terminal Care/Euthanasia? - Helpful Questions to Ask Ahead or in Hindsight
We now have a new dedicated End of Life and Bereavement Support Corner where we provide ongoing personalised community support for those that wish it. Since we are not part of social media, we can let threads run for as long as needed but we ask our members to please bookmark any support threads so we can keep each case to one single thread - it helps both sides, especially over longer periods.
But we are a friendly place which cares as much about the owners as we do about our piggies. We've done it for quite some time but this does formalise it a bit more. There is a need for a place where important questions can be asked and discussed in a mutually respectful way and where a helping hand/ear during a time of real distress can make a big difference.
End of Life and Bereavement Support Corner
I hope that this will help you?