What tweaks do slicers use to get rafts to stick?
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You could experiment with the 'cleaning' part of the startup gcode shown here

G1 X100 Y0 F4000 ; move half way along the front edge
G1 Z1 ; move nozzle close to bed
M109 S200 ; heat nozzle to 200 degC and wait until reached
G4 P10000 ; wait 10 seconds for nozzle length to stabilize
G1 E10 ; extrude 10 mm of filament
G1 z15 F12000 E5 ; move 15 mm up, fast, while extruding 5mm
G92 E0 ; reset extruder

I'm not completely persuaded that it helps a lot, but the idea is to fill the extruder (with some pressure from the bed at a 1mm gap) right before starting the skirt. Typically, I still find the first line of skirt may be quite blobby, but a 2nd line of skirt seems much better. Obviously you need the bed alignment right too. A raft seems to have enough 'extra' extrusion to mask these issues a little bit.