Doubles of this, or this plus a Clone, and then an instant-speed sacrifice outlet and possibly a soul sister. Too easy?

Apparently. :(

Is there an easy way to fix it? Probably either adding a payment or making it return the thing at end of turn.

Or adding "~ can't be sacrificed as payment" or "When ~ ETBs, if it was cast..."

"If you cast it from your hand". Unfortunately, that wording would upset a lot of people because you aren't letting them do the fun thing they want to do...

I think returning the thing at end of turn might be simplest.

Or perhaps rather than an ETB trigger make it a tap-and-sac ability on this?

Or just add a small activation cost.

Or just accept it? After all, you need a sacc outlet that does something and now it's a 3 card infinite combo - and wotc don't seem to mind those any more.

Better than a 3 card infinite combo. It combos with itself and any one card that lets you sacrifice a creature for net gain... which could be 50% of your deck, in theory. It still might be fine, though. Triple White doesn't have many good outlets for infinite sacrifice (though, artifacts often do...)

Vitenka's query does make me wonder if this would be so bad if it easily went infinite, especially if there were no enablers with it in Standard. After all, Melira makes any Persist creature go infinite for the same number of cards.

It is rare. At least it wouldn't pop up in limited with a crazy amount of frequency.