So many variables with cuttter quality, material etc and Each machine is different so much of it is trial and error so I don't put much stall in FSW.
I find to get gauge of the tooling and machines capabilty's it's worth sacrificing a cutter or two and some material by running tests at different DOC and Feeds. Your ear and senses along with machine will soon tell you if not happy. Bit of trial error and you'll find the sweet spot for your machine.

Don't be afraid to use larger tooling either I find 8mm works best in ali. Can cut deeper and make bigger chip which lowers chance melting. Reason you got melting with 5mm was chip thinning because didn't cut deep enough. Thin chips heats up melts then sticks to cutter. Thicker chip doesn't melt so easy and heat leaves with chip. Problem with smaller tooling is it's fine line between cutting right and wrong resulting in much easier breakage or gumming. It's also more critical you clear chips but at same time harder to clear chips when cutting deep due to narrow slot so loads of air/lube is needed.

Clearing chips and Lube/Cooling make big difference and looking at your Video you didn't have enough Air/lube to run those higher feeds with narrow tool, could probably gone bit deeper has well for thicker chip.! . . . Keep playing and you'll find the limit.? . . . . It's often much more or deeper than you think.!!