I've never used Affinity designer so I can't really comment on it.
Neither Inkscape nor Illustrator can create actual font files. You could use them to design/draw the glyphs however, but you'd need additional software to create an actual font file. Font-Forge is free and open source. I believe there's also a plugin (not free) available for Illustrator.
Nearly all vector software basically works using the same principles - at its simplest: Bézier paths with anchors (Inkscape calls these nodes) and curves. The main "standard tool" I suppose would be the Pen Tool (aka Bézier tool). Both have different shortcuts for some functions while using that tool, but the functionality is essentially the same. I use both Inkscape and Illustrator and don't have a problem switching between the two.
I don't think there is anything that would be analogous to learning to drive on the left or right when comparing Inkscape with Illustrator. They are lots more similarities than differences IMHO.
Ultimately Inkscape and Illustrator are a little different when it comes to what they were designed for. Inkscape was designed for SVGs, its own particular little niche if you like. Illustrator's niche is for print - it is a standard tool in the print industry. As for uses with cutters/routers, neither were specifically designed for that purpose, but some people do use them for that. I see no problem here with using either.
There are differences in the user interface, but that's unsurprising. The same goes for all software. If anything, personally, I find Inkscape's user interface simpler. Illustrator sometimes over complicates things. But this is only my personal opinion. Others will probably feel the opposite.
As far as creating outlines for cutters/routers, you could say that over reliance on effects or filters could be considered a bad habit - since you want vector outlines, and certainly no raster effects. But there is no obligation to use effects or filters in either Inkscape or Illustrator to get things done. These would not necessarily be considered bad habits for other uses however, such as when creating graphics.
Oh and Inkscape is free, so that last problem (payment) doesn't apply. There's absolutely no reason you can't have both Inkscape and some paid software. Two for the price of one!