Encrypting both MacBook Pro 16" 2023 and the first TM backup.

I haven't found encrypting my 2023 M2 slows it down and yes you can encrypt both. I did the Filevault first and then after formatting a Samsung for Sonoma I encrypted it, but you definitely have to save your passcodes to decrypt both. I haven't used a LaCie in years. The last one was a HD and that was like back 12 or 14 years ago.

My suggestion is buy yourself a good SSD and leave the HD for what ever. Apples APFS is meant for SSD's. I quote Joe Kissell.  "APFS does have one significant downside: because it’s designed primarily for SSDs, its performance on mechanical hard drives is terrible. I mean, almost completely unusable. (Fusion drives are better, but still not great.)" I use T7's and T9's.

Your Mac is already encrypted whether FileVault is on or not. FileVault just adds the capability to login and decrypt at the same time and ties the encryption Keys to your user login instead of auto decrypting