It's not a bad idea, specifically the 12 hour difference, as for the spawns it should only be applied to some legendaries that aren't possible to catch yet like the ho-oh/lugia idea someone mentioned and maybe the gen 6 starters since they don't have fixed spawns right now, it would suck to have restricted spawns like that for a lot of poke because some quests require dex data and imagine if you had to transfer to another server just for that? Some of us have been playing on the same server for years and being forced to switch would be quite annoying.
A way to solve this issue would be to reduce the transfer cooldown, instead of 30 days make it only 15 or even a week (i don't know if frequent transfers would be a problem for the devs), we can't do bosses on both servers anyway so i don't see what's the problem of being able to transfer more frequently, but unless it's a very good pokemon like a new legendary or a favorite of mine i probably won't be switching servers just for a few dex data and this seems like it would be more of a hassle than something beneficial.
Oh, and also the economy, i saw someone mentioned this and it definitely seems like it would be a mess to deal with the rising prices of pokemons and ccs because of exclusive rarity and being forced to buy and use a cc to transfer back if you don't want to wait for the transfer cooldown, we're already seeing the slight increase in price due to the current event and i wouldn't be surprised if the price went up way more than this if this change were to be implemented, that's not good for a new player and also annoying for older players.
When i started playing years ago cc was around 200k and new players had the chance to buy one without having to save up for a long time but now it's pretty much impossible and this change would only make it worse, and let's not forget that since the multiple accounts are a problem for the economy it seems likely that you guys want to force us to play on only one account and having the income cut by such a large amount would make it very hard for players who don't spend real money to feel like it's worth buying a cc and transferring just for a few exclusives.