The vast majority of the issues with the 705, 500 and now 820 are not solved by getting a new or refurbed unit. They're intrinsic to the software and hardware design.
And it won't change as long as Garmin continue to use the same platform for all devices. What works on one unit doesn't work on the others and totally screws them. At least Wahoo with the Elemnt and Elemnt Bolt have the units do exactly the same thing so shouldn't be nay issues with the firmware. But, the 820 is far too small screen size wise to be of much use with mapping and iConnect widgets.
Let's not forget either, that Wahoo's units are far cheaper than Garmin's.
No, they aren't "far cheaper". They aren't even always cheaper.
The Wahoo Elmnt Bolt is $250.
The Wahoo Elmnt is $330.
The Garmin 520 is the closest match to the Wahoo units.
The Garmin 520 is $300.
The ELemnt is more closely aligned to the 810/820 as the 520 doesn't do full mapping. Here, the Bolt is £175 and the Elemnt £219 far cheaper than the Garmins. If you insist on comparing price with the 520, it is £200 pounds here making the Bolt easily cheaper.