You can change the gears in the 2.85mm QR to 1.75. Contact bondtech for 1.75 gears and instructions. I don't know what the procedure is but it can't be trivial because a customer of mine who did it broke something in their feeder. But I imagine with the right tool it's not bad.
Wait you have a UM3 - you should have ordered the DDG feeder. Much cheaper! And simpler.
Also the feeders that come with the UM3 can print 1.75 or 3mm filament just fine with no modifications.
yes. That's all correct. You enable dev mode in the menu system somewhere. But I don't think you can change it while it's printing.
Once you do that the ip address should be displayed (if it wasn't already) and you can ssh in.
I recommend you mess with internet using conman as that is the preferred utility for this distribution of linux. Google about conman if you aren't familiar. You can disable wifi that way.
What printer/hot end do you have? 25mm/sec at 1mm width and 0.8mm height seems really fast to me. You're extruding a ton of material at that speed. Unless you have a really powerful hot end I could see the sheer volume of material dropping the nozzle temperature. When the nozzle changes velocity, the extruded material isn't hot enough and gets pulled/separated from the lower level which gives you your rounded corners.
I would set your print speed to something really low, like 5-10mm/sec and try again. If it looks better, your hotend probably just doesn't have enough thermal inertia to withstand the huge volume of material. At that point you can either print slower or print hotter.