Tc, I'm really sorry to hear that the implant has not worked out, I had really high hopes for you and thought you had finally cracked your absorption problem.
From what I have read on here, it seems that women with surgical menopause, ovarian failure or reproductive depression need very specific and targeted treatment and specialist help. I thought you would get this at the Chelsea and Westminster and I am surprised they don't have an emergency helpline even during lockdown.
You could try to get hold of some oestrogen to top up the implant - you said you have some Sandrena for now. Perhaps a combination oral and transdermal oestrogen like Alicess suggested would work. It has to be worth a try.
If you haven't already done so, I would use this time to do as much research on the internet as you can and if all else fails, find a specialist who is experienced and successful at treating surgical menopause. You could then sound them out and decide if you think they can help you and make an appointment, funds permitting of course. Private clinics are doing telephone consultations during lockdown and still offering ongoing support. Obviously it will cost money and only you can decide if it is worth it but it is an option.
Re the Mirena coil, it does have a drag down effect if you do not have adequate oestrogen. I was OK with it pre menopause but had problems once I hit the menopause and often had vaginal infections and thrush. It has a drying effect if not supported with a decent amount of oestrogen.
I'm at a loss as to what else I can suggest but I hope you can get some professional help.