Got it running yesterday.
I went for the second run with iPhones (6 @ iOS 12.4, 7 @ iOS 13) - it's slow, it recalculates but in the end it sends routes to the GPS. From time to time it chokes and displays error in the app, but after a while it appears on GPS unit.
Tried multiple routes with distance up to 120km and it was mostly ok/usable.
Went for android with my Huwaei and it wasn't great at all as expected. Sending was painfully slow, dropping connection many times - unusable.
Then I dug into system settings to make sure all Huawei's baterry optimisations are off and it turned out that these little fkers were turned ON.
Disabled all of them and... it works!!!
Sends the data much quicker than iPhone. From time to time shows an error that the BT device was unpaired but it sends the track to the GPS unit anyway.
What still worries me is the fact it always sends the route to starting point - even if you choose "NO" on the pop-up. I'll have to test that in the field with active gps signal, as all the above testing was done with 0 GPS signal.