If you custom built and got "everything" right you are one of the lucky few. I'm curious about your "quality of life" comment, is this due to location or other things in the house? It seems to me if you have a solid roof over your head, power and running water, feel safe in your neighborhood, the rest is all gravy.Now if you mobility issues that's another story.
The biggest quality of life (QOL) improvement is definitely going from unsecured, to a secured dwelling, which is available regardless of a custom vs pre-existing home.
Other aspects of the improved QOL also were available in an existing home, location is part of that, as is a smaller house.
Some aspects are not as easy to find, but can be renovated for. Such as all cabinetry under the counters being drawers, no carpets, expansive south facing windows (drastically lowered power bills and more natural light).
Some renovations can be as, or more expensive than building them to start with. Healthier radiant heat vs forced air run by geothermal.
Between the radiant heat vs forced air and no wall to wall carpet (very common in my region) our allergies trouble us much less, I sleep better, nap less, and just feel healthier.
And because the house is just right for us, I don't expect to be moving ever again.
All drawers under counter height, curbless shower, doorways wide Anouilh for wheelchairs, and one level capable living will allow us to comfortably age in place. We even constructed downstairs to be easily made into its own apartment with some renovations, so if we require live in help we can do that.
Over our life, we will avoid the cost, stress and time of moving.
We paid for it up front in extra cost and stress, but over our lives I expect it to be a net savings of both, even ignoring the QOL improvements.
The 'perfect' house is different for everyone. We had some very specific aspects of the house not available widely on the market (net zero energy and extremely low upkeep/utility costs). For others, they may find a house on the market that is perfect for them, or can be made so with less stress and cost than building custom.