IMO higher speeds are best because they require less torque and force for the same material removal rate. It sounds like you use the Makita’s highest speed, which is best in that regard. I don’t think its wise to use endmills not rated for the speed they’re used at. Carbide should last longer, is more heat tolerant, and is stiffer than HSS. But, everything gets dull eventually and different manufacturers use different grades of carbide, so who knows which brands last the longest. (Maybe you can do some life tests on different brands?) Do your endmills look discolored after being cleaned? If possible, you can minimize heating by feeding faster and more evenly distribute heat and wear by using larger cut depths with smaller widths.
2019-09-24 Update: @Joelbreaksthings
You might find this HEM Guidebook worth looking since it probably addresses some of your concerns about tool life and wear.