A middle-ground I think a lot of players might like is simply reticule / free-mouse swapping. This allows you to target via your character facing at will, so that you can still move comfortably while selecting a target, or target vs. a free mouse, whichever you prefer at the time, toggle-able with any given bind (I tend to make this my mouse's "Fire" key). You could even swap between two completely different key configurations in the process, including or excluding LMB, RMB, etc. from regular play. Given that I find the reticule more intuitive, especially when needing to target from among many mobs or during periods of movement, I'd probably spend most of my time there, but that doesn't mean I wouldn't swap back for healing, etc., especially if I could program a third intercepting set to allow me to cast LMB, RMB, and mod keys (Shift, Alt, Ctrl) for each on party or alliance members via direct clicks or the party list or attack spells via clicks on my target bar. At that point I'd rather my keys be auto-aimed at my current target.

Just some concepts I'd like to toss out:

Action combat doesn't have to be especially APM/IPM* intensive. Button holds can provide as much play unless allowing for changes to combos based on input delay (e.g. in Devil May Cry). By that matter, XIV could still have just as much nuance even if one were to simply hold down or hit once to automate "Thrust Chain" instead of hitting the skill 5 times, inserting oGCDs between. The issue is only tangential to either style of combat. FFXIV suffers from button bloat because it's been specifically designed as such, not because it's tab targetted. BDO has about as many actions in just 6 keys because it's smart about it, not necessarily because it's an Action MMO. You can have high APM/IPM, soft targeting, finely differentiated manual timing, AoE integration, and 4 hotbars worth of keys, for instance. Which then would you call it?

(*I prefer to consider APM as the differentiated actions a character takes per minute, and IPM as the number of inputs, keystrokes or clicks, that created that, but APM is the standard term for both, so have both.)

Action combat doesn't have to break the "holy trinity". While I would rather players actually have ways to skillfully outmaneuver damage, one could still have a satisfying amount of avoidable damage even while topping it off with enough unavoidable or uptime-reducing (potential) damage to warrant a healer. It'd just take a lot more potential damage overall, and may also warrant some QoL-like ways to deal with really poor players, such as each heal also tacking on some amount of bonus mitigation for a time, and may encourage a view of Support that's not limited to healing and MP/TP battery replacement alone. It just makes the decision more realistic. How much time can be saved by bringing a support. In some cases, the fight might not be clearable without it. Others, the added aggression possible for the remaining DPS outweighs the loss of one. But, you don't simply die outright to a composition check. (The same concerns of course affect tanks as well.)