I think the catch is it has added more for solo players, but it is either un interesting, locked behind chores or they feel like they can’t do it under the current set up, just using your examples:
Covenant minigames:
Kyrian=vehicular combat
Venthyr=a 1 time a week event
Necrolords=an NPC companion that can aid you out in the world and provides weekly quests depending on the ones you make
Night Fae=a fire and forget for the most part
Torghast had potential, but it does not provide gear and for the upper layers one might need such, yet how can they get it in solo content when the level that is recommended is heroic raid at the lowest? Oh, and any that say that Torghast does not need gear above what is available solo, alright, the ilvl may not be needed but what about a set with bonuses that are active only in Torghast?
Mage tower remix was overtuned at first for some classes and undertuned for others, but even after that was taken care of, there was a feeling of having to do older content to get the right gear to tackle it.
Maybe the issue some are having is not the quantity of solo content but the quality of it, in this case the past expansions might have had LESS content but it was at least the quality of a decent meal at a casual dining location, where SL did have MORE solo content, but the quality is more long the lines of fast food during a meal rush.
At best, SL has the most solo content yet, but it is uninteresting, at worst it has the most content players can do solo but in order to do such for a decent chunk of players, it takes gear from group content to do.
Just as possible changes to SL content:
Kyrian minigame: instead of vehicular combat with soulbinds, an option to take the trial your self or serve as a second to your soulbind
Night Fae: maybe a bit more involved like the farm and garrison were
Necrolords: possibly scenarios where you are fighting along side your construct in a move against the other houses or the big bad (last boss) of the patch
Venthyr: only thing I can come up with is allowing the court more often
Torghast would need at least torghast specific gear, ither with bonuses for it alone or a PvP like set up where as long as you were within Torghast the gear was higher ilvl.
Above all though, Blizzard needs to STOP tying progression to one path, like Torghast for legos, just so they can get the figures they need to warrant having it in the game.