Here's my tuppenth worth.

Huanyang VFDs are well understood and documented; and the collective experience appears that they are as reliable as you could expect. The VFD you link I'm not familiar with, though the manual looks comprehensive and understandable. I'm not sure quite why you're overspending the drive by 50% compared to the spindle - your money, your call. You can easily interface the linked VFD to any typical CNC set-up.

The spindle?, why air-cooled (rather than water cooled)? Air cooled have a reputation for being noisier because of the fan, and the cooling is less effective at low RPM. Water cooled requires a modest amount more peripheral devices (I use a small PC-cooler reservoir/pump, a 120mm radiator/fan and a couple of 8mm pipes - self contained - and the whole lot (including Spindle) runs quiet enough to have an easy conversation without the howling banshee that an equivalent wood working router would present. Air cooled does offer the issue of cooling surfaces becoming clogged with fine sawdust if wood is your material of choice. Plenty of wood-cutters on the forum that will dispute that, of course.

I don't have a 1/4" DeWalt as a comparison, but my first MD machine came with something like a 800W hand router - the best thing that ever happened to that machine was the spindle burning itself out. I replaced with a 800W water-cooled spindle and the difference in experience - both audible and performance was night-and-day. Many people, myself included, chose the 2.2kW flavour for the benefit of the ER20 collet system. If you're just using 1/4" or 1/2" wood working bits then the benefit is less pronounced.

For the VFD and spindle - you don't mention what it is that you're looking to machine. The spindles aren't particularly good performers under 6-7k RPM. If you're making sawdust, rather than man-glitter then you'll be operating much closer to the higher RPM that these spindles operate very well at. And you'll have a better operator experience with the WC solution.

Inductive sensors?, my £5 sensors have never failed me, and appear to offer reproducible results. I'd hate to crash a bed into a £150 one, during commissioning.

There's plenty of ways to introduce unreliability into your system - VFD, Spindle and cheap inductive sensors don't top the list.

Just my limited experience.