Thanks for the photos! It looks like a worst case scenario, you are running a real deep cut at full tool width (full WOC) with a less than rigid spindle, and probably poor chip extraction and too-high RPM thrown in!
When cutting out a keyring (say an oval shape) try cutting around the outside exactly 0.5mm too large on all sides. Cut down in much smaller depths of cut, like 0.5mm or 0.75mm DOC each pass. Definitely keep it under 1mm DOC.
Keep running at 900 mm/min as above. Go down to about 0.3 above from the table, which will leave a "skin" easily strong enough to hold the part in place for the final operation.
Then for a final outer cut, cut at the proper size which will be a 0.5mm width of cut. Conventional or climb, whichever seems to work better with your tool. Come down at 1mm DOC per pass, but stop at about 0.15mm above the table, this should leave a "breakout" skin and 0.15mm works pretty good for small parts in acrylic.
Also you might want to try a mister bottle like those small handheld misters you get in the supermarket, people use them to mist plants. Just plain water and a very fine puff of spray every now and then when it's cutting. Combined with the lighter DOC this should end all your melt problems.
Here's a build thread of my DIY plastics mill, it gets a lot fo acrylic use;
DIY hobby small plastics mill/router