I'm too late for this to be directly useful to you now, but I'm tossing these out there for anyone coming back to this thread later.
For 55+ there's two old instances added in DoN that most people don't remember/never knew about in Unrest and Hate's Fury. The missions scale to level up to 70 (end game at the time). Are they any good? I don't know. I haven't been in them in well over a decade. But they exist.
Savil Tranquilsong gives the Unrest instance.
a shady merchant gives the Hate's Fury instance.
(There's also an old
Befallen instance for even lower levels added at the same time, but I never actually did it, so I'm not willing to say more than 'it exists')
For 75+ both Gyrospire Beza and Zeka have instances from
Clockwork Warmarshal. 6 instances each. Non-respawning, if I remember correctly, which isn't ideal for this this, but you're all alone until you're out of mobs. Mob difficulty/exp is the same as the base zones, just in an instanced version.
You already mentioned the
Mechamatic Guardian instance, but it's worth repeating because you can hang out in there with respawning mobs for a long time.
And of course there's lots of instances in SoD itself for missions, but they're not really great for straight grinding exp, honestly.