Hmmm this is tricky
@Sword of Spirit , we don't want to give away the abilities of the 8th level Etherealness.
I had forgotten it was 8th rather than 7th level. Such...a...bad...spell. In any event, you're right, I don't want to obsolete it.
But I could cast Ether Conduit, get on the other side of the Wall of Color, then just step back into the curtain. (As far as I know, there is no spell requirement for entering a Color Curtain
Absolutely!
or stepping out of the Border into the world.)
This, alas, one can't do. When inside the Border Ethereal you need to have some sort of special ability to just step across into the bordering plane, which this spell doesn't grant you. (Actually, nothing but seemingly some monster abilities or
gate does let you do that.) It spell has a 10 minute duration, so you can pop back to the Deep Ethereal and go back and forth through that conduit to your heart's desire within that 10 minutes. But that doesn't grant as much power as it seems at first glance.
There is a balance and a constraint. The balance is that it can't actually replicate the intended use cases of
etherealness, and the constraint is attached to why it can't do that.
Etherealness is basically undetectability plus phasing through walls and objects. It's best uses are for infiltration, escaping combat, or scouting. Its limitations make it non-ideal for anything else.
The important fact that prevents these uses for
ether conduit is that you don't appear precisely where you want to. Each location on the Wall of Color corresponds to a location in the Border Ethereal, which overlaps precisely with a location on the bordering plane, and only 70% of the time does the location of the other end correspond with your casting location. I'm implying something like the way the PHB describes the
plane shift spell without a teleportation circle as a target: "If you are trying to reach the City of Brass, for example, you might arrive in its Street of Steel, before its Gate of Ashes, or looking at the city from across the Sea of Fire, at the DM's discretion." While my intent with "corresponding vicinity" is that you don't go too far afield, you are almost certainly going to be dozens to hundreds of feet away in a random location--and 30% of the time you are going to be 2 to 200 miles away. The spell also has a 10 minute casting time.
The 10 minute casting time means you can't use it for escaping combat. The unreliable destination means you can't use it for infiltration, because even if you end up close to your current location and travel to where you want to return to your original plane from, when you cast it again you're not going to get exactly where you want to be. Forget about using it to get into the vault, etc. You
can use it for scouting, but all of that travel is going to be a big hassle and gamble. You can easily roll poorly casting the spell to pop back and forth a few times in a row and end up hundreds of miles from where you wanted to be. Skirting the Curtain can help you get back to where you started half of the time, but each time you cast the spell you're still potentially 200 miles off.
Given those limitations, I lean against having a duration of stay. The point of this one is to get you into the Ethereal Plane, and then let you go wherever you want from there, but not be useful for the things
etherealness is designed for. It's like a more limited, lower level
plane shift.