Honestly I think the effect third parties can have on Ukraine is significant but the effects on Russia are not. Not unless there willing to commit troops, Russia can after all just force Ukraine to accept if things keep going the way they are going regardless of any third parties opinion. Sanctions relief well desirable are not strictly speaking nssesary. After all economically speaking Russia is treading water right now if they force Ukraine to surrender they may still face sanctions from a angry west but they won't have wartime costs. Sanctions will eventually be reduced as Russia exports well energy food and minerals. Demand and a discount will in peace time reduce sanctions naturally
In that regard the question isn't what peace deal third parties find acceptable, it's what can be offered to entice Russia to dial back it's maximum goals.