I'm about to switch from a Tangem wallet to a Coldcard Q. Tangem was my first ever HW wallet, so I don't have any experience "changing wallets". Is it better practice/security to generate a new seed phrase with the new wallet, and then send my BTC over from Tangem (testing with a smaller amount of course), or should I just restore my seed phrase from when I created the Tangem wallet?
Don't reuse your wallet addresses, use change addresses.
Don't resuse your wallet seed phrase when you change from one wallet brand to another wallet brand or software.
Create a new wallet, with new wallet seed phrase on a new brand or software, and move your bitcoin from current wallet to this new one.
https://bitcoin.org/en/secure-your-wallet#backup