I wear the R24 and record the heart rate for the swim. Then I export the R24 .fit file to email via the Scosche app and merge it with the manually downloaded Garmin .fit file using FIT File Tools (free). Finally, upload the merged .fit file to Strava, TP or whatever. Strava doesn't show second by second heart rate details but does award an average effort calculation so your 'fitness and freshness' gets updated accordingly. TP also awards a TSS for the swim. Hope this helps.

I experimented with every Fit File Tools trick I could muster, but as I commented, "I have not found a way to effectively merge an HRM file into a FIT file captured by another device." The problem as you found is that you can combine stuff, but it is not fully readable by anything (Garmin, TP, Strava, etc.). Some of the analysis can see HR data, but it cannot see it in context of the workout. I wanted to see my overall HR and how HR aligned with my intervals. No merge tool I found could accomplish that.

I found this problem as well. I even tried the highly underrated Polar OH1 and I couldn't merge anything. It's been my go to HR for running and cycling and only use the Garmin Swim for obviously swimming laps and outdoor.

I have the Scoche 24 and freaking love it. Way more comfortable than a chest strap, fantastic battery life (I charge it once a week at most), very accurate. The only downside I see is the aero penalty on the bike while racing. For that reason I don't put it on until the run. But with a power meter on the bike I've never felt like I needed it then.

I also got one for my wife (non triathlete) who does a lot of HIIT style exercise classes and it is working great for her as well.