What is a good way to know what level a spell should be?

Recently, I had a interesting thought for my campaign.

There are a handful of powerful magic users in the setting, and I was thinking it would be cool to give them their own spells (The way Mordenkainen, Jallarzi, Bigby, Tasha, Ottiluke, Otto etc have spells named after them).

One of them, "Palus's Endless Bog", (Palus is an Arch-Hag who lives in the swamps btw) creates an area of Difficult Terrain (Idk how large yet) that forces anyone who's in it to make a CON save or have the poisoned condition [Spell requires Concentration for 1 Minute]. I'm thinking this would be a level 3-4 spell, but idrk much about homebrew spell balancing so it would be great to hear some advice

Yea the spells that are most similar to it are 3rd level. Hunger of Hadar is pretty similar, Difficult Terrain, concentration and an aoe. The only difference is that it deals around 4d6 damage and blinds people inside of it.

Poisoned immunity is common so I think it’s pretty balanced, if not on a slightly weaker side, depending on the aoe size.

Comparing it to official spells is the easiest way, unless it's something completely unique. Then you just kind of have to go by feel and hope for the best.

Compare it to existing spells. In general, at a given level of spell there is a trade off between: (1) area size / number of targets (2) damage (3) conditions imposed. If the spell covers a larger area than a published spell it should deal less damage, if the spell inflicts a worse condition it should target fewer creatures / smaller area, etc...

Maybe depending on how much damage it deals or its affect?