So let me explain about Garmin… I have said in few other related Garmin topics but I can tell again.
You have 2 different things: list all your activities into your Garmin account to track what you do in one place (kind of Strava stuff) or get the training metrics from Garmin (which is the main point of a Fenix or other watches having that metrics).
So, double recording is a must. You simply have to. And no you don’t lose other information, if you follow some steps.
So first let’s start with rowing as we are in Rowvember anyway. I assume that’s what you do with your C2? You have few options: use Garmin Row activity, use Ergdata App on your phone or use ErgIQ on your watch. I row like this so I know the subject haha
In order to get the training metrics you will need to record your rows on your watch with ErgIQ. Connect on your PM5 first the HRM with ant+, then ensure FE-C is activated (the PM5 will tell you that’s the option for newest Garmin watches), then open ErgIQ on your watch and it will ask you to pair with the PM5 say yes on your watch and if it’s good you will see your HR displayed on your watch.
Row your session, and close the session on the PM5, while observing your watch, you will see a very fast message “Saving…” and then back to main screen. It’s done and you have your rowing session uploaded into Garmin Connect.
OK now Zwift uploads: it’s nice and everything to connect Zwift to Garmin so you have the Zwift activity available in Garmin Connect. It does NOT have any training metrics (the Garmin ones). Recording with your watch will have training metrics but no GPS trace, incorrect distance and 0 elevation.
Fitfiletools website is nice but can’t do anything about that. You need FFRT to merge both fit files (and I have been telling that so often now that I should be sponsored by FFRT lol). This tool will allow you to take the data you want/need from Zwift fit file and merge it into the Garmin fit file, which will overwrite distance elevation and whatever in order to give you the GPS trace, elevation, identical distance from Zwift, but keep the training effect TE and other metrics. You can then delete manually in Connect the recorded activity with your watch and upload manually the fixed fit file, which will keep the training metrics as you wanted them => purpose of buying the watch. You could then also delete the Zwift activity from Connect as you wouldn’t need it anymore or simply just leave it there, it doesn’t matter and has zero impact (only on the total distance stats but well, you can live with it lol).
I hope I could help 