Harbor freight bench top mill

Use wd40 or kerosene to lube your cutters , spray misters work well also . Aluminum builds up on the cutter edges causing many of your problems. Two or three flute endmills will do great buy the best you can afford , remember high speed and cobalt cutters have worked for fifty plus years.
 
End mills that do a nice job in aluminum look like this. You can get them in most machinery supply places, even Ebay & AliExpress. (With the usual offshore caveats) I've found prices better on Ali, especially carbide which are razor sharp. Unfortunately Asian are mostly metric shanked so I have an ER collet system which has paid for itself very quickly.

Nobody has mentioned climb vs conventional milling direction yet so I'll throw that out. Simply plunging & traversing in one direction means you have the cutting edges going in opposite direction relative to either slot side. If one surface of your slot looks ok but the other is raggy, this is why. For something as simple as a bolt clearance slot, make the roughing slot, then step over a couple thou & finish in climb direction only. In this mode I find 3 or 4 flute cut very well. Maybe this is part of the confusion. Now your slot end radii will be ever so slightly out of round because of this slight step over, but again for a clearance slot it wont be seen or felt. Sometimes people drill a slightly oversize hole on the ends for this reason, it has a bit of dog bone appearance but cosmetically better.

For more precision work, if you need to hit exactly 0.250" slot and with an excellent finish, then its better to step down to an EM diameter beneath 0.250 & mill as above.

BTW if you haven't discovered roughing end mills, you really should. They are not just for industrial hogging, they are excellent on hobby machines because they cut chips so efficiently & actually work better with lighter less rigid machines. I typically have a matching pair like 0.250" roughing & 0.250 finishing. Hog off the bulk material & then swap to the finer EM for finishing. Because the EM diameters are the same (nominally) you dont have to change your dial setup.

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This may not be appropriate for your project but hopefully a picture makes things clearer. Here I require exactly .250" width AND the best finish. But I don't necessarily care about the end diameters matching up exactly. Drill (oversize) 9/32 ends, slot & finish with under size 3/16" EM. Now you can use the appropriate milling direction to achieve the finish.

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