I know of one answered question on a similar subject, but mine is not quite the same thing.
Berkeley's idealism holds that to be is to be perceived---nothing can exist, or can be thought to exist, without something perceiving it. An unperceived thing is literally inconceivable. The world remains extant when we're not looking because God is looking, all the time. We get to keep our idea of a permanent world, and multiple people living in it, by this move.
Kant's (considerably softer) idealism holds that we only have access to phenomena, but we know noumena absolutely must exist. I'm not solid on this bit, but phenomena are sort of "wrapped around" noumena? Causality is a property of phenomena, but phenomena in some way spring from noumena. Maybe noumena are the material substrate Berkeley rejects? In any case we somehow get to have a permanent world, and multiple people living in it.
I'm not sure if anyone notable subscribes to this view, but a third option, solipsism, is that there is nothing ensuring the permanent existence of things, and the world really does stop existing when we're not looking, and nobody else has any subjective experience.
My question is, is there any kind of fourth option? Is there any way to be an idealist without believing that the world is only sustained by God watching it, and also denying (or, like Berkeley, finding incoherent) the idea of noumena or a material substrate? And can we hold this belief without falling into solipsism?