This kind of sign? Simple pantry sign
The 1/8" flat cutter should work fine, as long as you make sure all the inside curves are big enough to accommodate that bit. I’d suggest slowing down to 1/16" DOC, 45ipm, with a 22.5ipm plunge, 30krpm. The smallest width of my sign was 1/10" so 1/4" shouldn’t be an issue. Running a 1/32" offset from your vector for roughing using conventional milling, then a finishing pass using climb milling would also be good. For the finishing pass you can do 1/16"DOC, 135ipm, 60ipm plunge, 30krpm. Basically, for your finishing pass you’re only cutting 1/32" wide material, so you can really go a lot faster and get nice finish.
As for work holding, masking tape and CA glue are the only way to go, just make sure you put a couple of beads of CA glue ALL the way across your material. You want to make sure some CA glue ends up on your project piece after all the waste is removed. Tape/CA glue is great but it takes some planning to ensure all the pieces that need to be held down are still held down after you cut away the rest of the material… and some of your bigger waste pieces are still held down and don’t go flying off when you cut through lol