Would you consider graft attempting it again?
Personal thoughts on rmj.
1) There are definitely preferred micro-climates where rmj grows hardier with healthier and more compact foliage. I am based in LA county, rmj here can grow healthy but its finicky and the slightest mishap or lack of vigor results in floppy, juvenile, and weak growth. I don't know your climate area enough, but I would say first is to evaluate how healthy you can cultivate rmj foliage there.
2) A way sometimes to evaluate very old bonsai is age in cultivation. This is not necessarily the physical age of the tree, but the number of years successfully cultivated as bonsai. Having apprenticed in Japan, and getting many opportunities to see old trees the difference in person is night and day between a tree with just a good aesthetic and one old as bonsai. The idea is that say (arbitrarily) for a 20 year window, you want work over that time period to appreciate and be built upon previous work. But if your tree requires major reworking every few years, health related, or by nature of growth habit even if you work on your tree for 20 years its age in cultivation will forever be 3 years old. Case in point, there is no maturity of the design since 2021 and if anything as you already mentioned had set backs related to health.
Rmj naturally is a very lacy, floppy foliage. It is not very turgid and is a slow process to develop structure on, especially if the tree is always being styled with very weak skinny branches. Short of a very clean design and great health, more often than not rmj require major reworking every few years which resets the age in cultivation calendar as opposed to building upon previous work. I am sure Bjorn understands this, which is why he chose to graft the tree.
Success rate of your scions will relate to health of understock (the rmj) and then your donor (scion, chinensis). I've observed the same issue where an rmj I took over from a local professional was graft attempted and had the majority of them fail. Issue was that the tree was just not strong enough. It took me a full year to rebuild strength to a state of vigor (from an almost dead tree) and I grafted this year--seems like all will take.
I did watch Bjorn's video before and he wrote off the failed attempts as a more poetic sign of retaining the native foliage. But my personal feelings are that bonsai is already so far removed from its natural state that unless the native foliage is very impactfully conveying a certain aesthetic while being cultivatable for bonsai trying to keep it for the sake of a natural look feels hypocritical. Its people who assign aesthetic and feelings to nature, while in the natural environment trees are only concerned with survival and the easiest accessible way to grow.
I believe there is a fine line between representation in nature and then cultivating the human element on our trees. Just as yamadori accrues age and character in nature from harsh environment and slow growth rates, in a sense we can impose our own history of time on the tree. Trees that successfully carry this, in part with the natural character, are usually shoe in show winners are major exhibitions.
I think situationally native foliage can look great, but I think this is not one of those cases and if you can grow the tree to a state of great health then graft it, this tree will have a brighter future as bonsai.
