From what I understand, the price difference is small between the RTL510 and the RTL515. If you may want to use the Garmin radar with a phone, the RTL515 supports BLE (Bluetooth Low Energy) where the 510 does not.
I bought the Garmin Edge 1030 Plus primarily for radar, mapping, and cadence display. One of the primary considerations for me was, even though I have a USB port available, not to run down my cell phone battery. I also don't want my iPhone 12 Pro Max exposed to the elements. If you don't need any of the other features other than displaying the radar, I'd look at the Varia radar display unit:
buy.garmin.com
