If a player without the DLC plans a team with a Stellar Tera Pokemon, I feel they can just go to any Pokemon-focused forum and they'll find someone who'll help change its Tera Type to Stellar. And even if they were to have a special Stellar Rayquaza Raid where you could get Stellar Shards (though I wonder if they can just program it to give Flying and Dragon Shards), it would still be a grind to do and against a powerful Pokemon to boot (and another disadvantage to not having the DLC: no Glimmering Charm). You'd have to really work to get those Stellar Shards, and it'll all be probably just for one niche strategy. So, yeah, I don't think "gotta gatekeep the Stellar Shards" is a good enough excuse.