mass deleting messages from MacBook air, or another solution?

Re: Any clever solutions? The TD;DR-- how so I easily delete a ton of Messages from my MacBook

but keep them on my iPhone?!?

iCloud is mainly for Sync of files across devices. delete a synced file and it deletes everywhere it is synced.

Apple suggests we: Archive or make copies of the information you store in iCloud - Apple Support

Note: If you use Messages in iCloud, deleting a conversation on your Mac deletes it from all of your devices where Messages in iCloud is turned on. See Access your messages on all your Apple devices.

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Re: mass deleting messages from MacBook air, or another solution?

You can easily delete whole conversations, if deleting messages is too laborious.

Seems you are using MacOS 12.3 Monterey on your MacBook Air:

(If you have updated to Ventura, change the Mac OS version at the top of the article below for accurate info.)

Apple support's article

Delete messages and conversations in Messages on Mac - Apple Support

says:

Delete a conversation: Select a conversation, then choose File > Delete Conversation. Or if you have a trackpad or a Magic Mouse, you can also swipe left with two fingers on the conversation in the sidebar, then click the Delete button 

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If needed: Messages support website (info. / support / contact options)

Messages for iPhone, iPad, Apple Watch, and Mac - Official Apple Support

All the best :-)

Wondering if most of the thousands of conversations in Messages are just "synced" via iCloud from the iPhone ... if so you could just turn sync of Messages off for the Mac only.