@Fraser It looks like that decal supplier is a good one to know - they can probably produce anything one is likely to need.
My P-Touch is the D600, which has a QWERTY keyboard and USB PC connection. I used the PC label designer to run off the FLIR label. Like you, I have a goodly collection of tapes but almost all of mine are 'compatible' rather than Brother originals. They seem to work very well indeed and are significantly cheaper than the OEM tapes.
For day-to-day stuff I use an extremely elderly P-Touch D75 and 12mm black-on-white tapes. It's easy to use and the tapes are good (and cheap), though you're very limited in message length and there's very little flexibility and no connectivity.
<edit> After seeing the genuine decals, I had a quick further play. Now all I need to do is work out how to make nice rounded edges...