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'Zillions, on a modern computer, plays chess at, perhaps, Elo
2150-2200.'

That might be true against Humans in 5-min games, but I have strong doubts
that that has any bearing on how strong it is. Based on Zillions
performance in Gothic Chess I estimate its rating more round 1800 Elo.
When I had micro-Max play on the ICC server against Humans it also quickly
converged on a rating of 2260, but it seems that almost any computer
program, no matter how poorly it plays, does that. I once saw a report of
Tord Romstad, about his attempts to reduce the playing strength of his
engine Glaurung. At some point he had crippled it so much that it would
litterally score 0% against TSCP (an extremely weak engine, which
micro-Max would totally crush), and blundered away a piece or rook nearly
every other move. It still kept beating 2100-rated players on ICC!

Anyway, Zillions might suit your needs, it definitely does not suit mine.
I need computer opponents to test my engines, so it is not really relevant
how many variants Zillions plays, just that it plays the variants for which
I have built engines. And if even the most basic engine I build totally
crushes it, I can learn nothing from playing it against Zillions. My
reason for playing engines against each other is to test which one of
different versions is better. If all versions score close to 100%, I can
learn nothing from it. And if I would want to play a variant against a computer myself, I might as well configure Fairy-Max to play that variant.

Of course even if Zillions were strong enough, it would still be useless
to me, as it cannot play automatically against other engines at all...