I feel like this is not a valid argument as you can simply unban tera and restrict the type to stellar.
Saying "this is not a valid argument" doesn't make it not a valid argument. Implementing Stellar would be pretty much 2 complex bans at once. It'd basically change mono from having a single type clause, and tera ban. To "single type clause with the exception of turning into a stellar type" and "tera is banned apart from Stellar", every tier very much tries to avoid these complex bans and I'll agree it makes no sense to have an exception made at least in my opinion.
To add to this. Allowing tera would mean we have to ban Terapagos, it's stellar form would be way too cracked to keep in and we can't just say "this one mon isn't allowed to tera but every other mon is". It has multiscale in it's base form and 160/110/110 bulk in it's stellar form with a 130 special a.
The effects of stellar terra are as follows:
What the Stellar type does is give a special boost to Same Type Attack Boost of your move. Every move of a type gets a small boost for the first time that type move is used by the Pokémon in battle. This is in addition to any STAB that the Pokémon has.
If a move doesn't match the Pokémon's base types it will get a 20% boost in the damage for the first time that move's type is used. So if a Galarian Zapdos uses Throat Chop, it'd get a 20% boost for that first usage but no extra boost for any other usage, and if it then went to go for Knock Off, Knock Off also wouldn't get the boost.
If a move matches the Pokémon's base type, then like standard Terastallization, it will get a 100% boost bringing it to double, much like when you Terastallize into the type the Pokémon already is. After the first use it will return to its standard 50% boost.
It's on pokemon like Iron Valiant where we'll see that a 20% boost on nonstab, on top of other items, is a hell of a lot, not to mention the extra power to stab, even if it's a one time thing this'll be more than enough to push pokemon over the edge. SD Pao is my first thought there, as the boost from tera means it's a favored ohko roll on corv at +2 after rocks. I don't see this being implemented without turning some potential suspects into guaranteed bans while some less threatening mons become more dangerous.
Personally I think the complex ban portion alone is enough to keep it out of the tier, but the fact it guarantees that mons that would otherwise be fine like terapagos need to go is where I really see it as an unnecessary implementation to make.