Hi Felix,
I'm also having a bit of trouble working AO-85. My equipment for FM sats is an arrow antenna, FT-817ND for RX and FT-2DE for TX.
With this I can work SO-50 without any problems. Whenever more that a couple of degree's over the horizon, I can hear it well and I hit the transponder perfectly. In fact, the UHF downlink helps a bit in pointing the antenna, and I would usually point it by ear, having just a mental picture of the pass. If you can hear well the downlink, then you can get in well, because your uplink has wider beam and much more power.
However, with AO-85 it's a bit hit and miss for me. I find it a bit difficult and random to hit the transponder, and I think it would be almost impossible for me to work it half-duplex. I will often retry and tweak things if not hearing myself on the downlink properly. The downlink of AO-85 is very strong, and it can be heard with the antenna all over the place, even pointing into the ground sometimes. So you have to know well where the satellite is before transmitting, and listen yourself and tweak the antenna as you transmit.
Several people report, and my experiences agree with them, that the receiver on AO-85 is a bit deaf for some unknown reason (this has nothing to do with the 70cm band) and that it's 5kHz low in frequency from the published specs. Thus, I will start a pass on 436.165 and work up from that as the pass progresses, being at about 436.175 at midpass and 436.185 at LOS.
Regarding working full-duplex, if you're going to get a single radio to do full-duplex, check very carefully that it won't suffer from desensing. This can be very critical, especially when working V/U. If you already have a UV-5R, I would probably get a second of those, or perhaps another handheld with better sensitivity to use as RX if you want to invest a bit more money. Of course, you'll need a duplexer to use the elk with two radios.
73 and looking forward to working you on the birds,
Dani M0HXM/EA4GPZ.