I bought my wife a base storage mini with this assumption and her iCloud messages was 95gb because of all the pictures and videos sent. There is no way to offload it or move it to store on my nas, or an external HDD that I could find
It's a very intentional thing to try to make you need more space.
I pay $30 a month for the 6tb iCloud plan and could find no other workarounds other than logging her out of iMessage - which is absolutely rubbish
If her iCloud backup is 95gb then it only costs $3/mo. for the 200 GB plan [1].
If you're paying $30/mo. for the 6 TB storage plan then that's because you're choosing to store a ton of stuff there. It's certainly not because of your wife's messages. And you're making my point for me -- I do the same! $10/mo for 2 TB. It's great.
Cloud pricing is pretty decently competitive when you compare it to building your own storage with the same reliability, which is going to involve redundant on-site hard drives in a NAS, and an extra off-site backup.
You can also just delete the largest iMessage media files directly from the phone, there's literally a feature for that. Since most people don't really care about saving any of the videos or images after they've been seen, or you can save them individually to your Photos at the time if you do care. I do a mass-delete every year or two.
We pay for iCloud regardless because we've crossed the 2tb threshold for our devices and our kids devices backups + photos and videos.
Deleting the iMessage data off the device is not feasible after like ten minutes it was showing me 20 at a time that had small sizes relative to the amount left to delete. Thank you for your attempt at helping though
Regular users keep their photos and videos in the cloud these days, whether iCloud or Google Drive.
And video editors are going to be using external drives anyways, the internal SSD is just for scratch.
If Macs were used more for gaming then that would be a major reason... but they're not.