I’m going to say this sounds odd, given the astral isn’t “solid” in the first place.
What one person experiences doesn’t always mean it happens that way for everyone. Magick is very personal because our energy systems are all different.
Personally I use temples immediately. My best advice is try it yourself and see how YOU feel about it… if you get feelings like it needs more visualisation to feel usable, (which is what I guess that user meant?), then go with that and meditate on it more.
I would also say the act of using it make it more and more real to you anyway. Practice makes perfect.
I’m not entirely sure there’s a difference. The astral is the mental plane, it’s your mind, and as you explore you start to see that areas of the astral are part of the collective minds of many people, until you find places that are the collective mind of humanity as a whole.
So in my view, to say something is imagination then means something real: that this is a thing that is in your personal part of the astral and not shared by others.
Either way, if you go look at it, you will see it. If you forget bits of it they can sort of despawn.
If the difference with imagination is the astral location making it more or less accessible to other consciousnesses, a temple in only your very close personal astral or imagination will be as static as you make it, with little spontaneity or unexpected activity from others, so you have complete control.
This is my preferred location for a temple: only those I invite can find it because it’s very close to me and I may have warded it, but even so I’ve had issues with astral entities showing up, or finding a spot I found was already in use by entities and active. It’s actually better imo to imagine your own not find one to avoid having others randomly wander through.
But as soon as you invite others, or, as here, tell people about it’s existence, then it now exists in our imaginations as well, and hence is no longer in only your personal astral. It’s already “solidified” because you told the Internet about it If you want to make a temple that is for you and only you you can’t share it, it must be a secret.