At your age, with no kids, I think divorce is your best bet.
I know a couple in their early fifties in a similar position to the one you describe now. They’ve been together since their twenties. Spark fizzled out in their thirties and they split up, one bought the other out of their flat. It was the asexual one who instigated the split incidentally, they thought it wasn’t fair on the other one, didn’t want them to miss their chance to have kids etc.
The sexual one even got married to someone else, and when that ended in divorce lived with another partner for a couple of years. The other had a long term live-in relationship (no sex in the latter though, just affection and emotional support).
But they stayed very close, absolute best friends. To be honest I think that always got in the way of their other relationships.
In their forties decided they wanted to get back together. To grow old together and look after one another. They were past the stage where kids might happen and knew they were never going to find anyone else they lived or liked as much.
So they split up with partners, bought a house together, got married (to make things like pensions and inheritance easier if one died young).
The one who still wants a sex life has permission to go outside the marriage. They tend to go for relationships lasting from three months to a year, with someone who knows about the marriage. This made easier by the fact the asexual one is very career driven and for the last six or seven years has been working abroad a lot, setting up an overseas branch for their company and working there. The asexual one also has permission, and actually had a few one night stands at the beginning, but that fizzled out pretty quick. I think they did it mostly so the other wouldn’t feel guilty to be honest.
The temporary partner is welcome to socialise with the long-term couple. Nights out, holidays together, trips to see the person working abroad (glamorous location and high salary) sometimes even living in the marital home.
It works pretty well for the married couple. Not so sure about the temporary partners though. There have been a lot of hurt there, sometimes because the kind of person who’ll accept that set-up often has some issues of their own to start with, or is naive enough to think that they’ll eventually be included in the same level or even that the couple will split up, which is never going to happen.