After trying a variety of tooling companies we settled on LMT Onsurd and have been using their Marathon coated compression spirals for several years with very good results, much better life time in the particle, mdf core & HPL we mostly use. I don't see any reason to go to collet chucks smaller than an ER32, unless you have to reach down into tight spaces with an extension. You will end up buying quite a few extra tool holders so you can leave additional bit set up ready to run.
The "pure" (no filler) plastics we cut with O flute HSS. We occasionally make tool boards for the railroad with either 2" or 3" UHMW and a 3/4" x 3" HSS bit. Cuts great but when edge shaping the boards makes a terrible mess. Chips fly 30' and are full of static electricity, so stick to everything. Just finished a stack of 2" now have an order for 3" boards. Lot of chips!
Aluminum cuts easily with a standard HSS 4 flute end mill. Some of the alloys tend to be gummy so you will need to use an aluminum cutting fluid. If your machine is not equipped with one and you do a lot of aluminum you should buy an add on system, pretty cheap. Otherwise you can just use a squirt bottle.
On our pod & rail machine we have a C-axis which I have gotten addicted to, even though there can be other ways to get there, just take more time. Having the 4th axis and the aggregates allows some pretty nice solutions that are either a lot more difficult or impossible on a 3-axis machine. Did you get the 4th axis, any aggregates?
Have you considered using heat shrink tool holders? You probably already have a torque wrench to set the collet nuts.