@JoelP wiring issues can be a nightmare to solve, especially by description, and even more so without a diagram. I'm sure VM has a diagram...
Your tach has a wire that runs from the coil directly to the tach. It also has switched 12v and a ground. I've attached the gauge wiring diagram, and regardless of what harness is used by VM, the gauges have to be wired this way, with the exception of turn/hazard indicator lights, and we wire our GPS sender to 12+ (constant) so that the GPS sender doesn't have to relocate a signal every time you restart the car.
Back to the tach, they have a delay in readout, but it is <5s. Any longer indicates an issue. Most common issue is a ground problem, and the 2nd most common, which we're all seeing more recently, is a failing distributor. More specifically the internal electronic pick-up. Again, not saying this is your issue, but from what I've seen it shows itself as a bad tach signal that is intermittent and directly correlates to heat and then failure when hot. However, rereading your post, I'd lean more towards a main ground issue overall, especially since you are losing multiple items at once. I'd be looking, and looking again, for anything that could have been knocked off when doing your switches. A wired pulled too tight, loose crimp at the wire, loose spade terminal, etc... especially since your were under there working and things worked previously. I say this from experience, as I have done something as simple as just heated up some wiring and the change in tension, direction of pull, any disruption, has revealed some other underlying issue, like a bad crimp/solder/whatever... Retrace EVERYTHING you did. When you did your timing, where did you power and ground your timing light? If you ground to the battery terminal did you knock loose an already loose battery ground or reveal a bad battery cable connection. Regardless of what wiring harness VM uses, a disconnected or loose main battery cable (power or ground) would kill everything... if you're going to resolve this yourself, just go through everything methodically and look at EVERYTHING you were in contact with, and then stuff you weren't... That's the best advise I can offer.
Regarding "tech for techs sake..." I just want to say that in the case of these VDO gauges that isn't the case. VDO (nor any major manufacturer) would agree to make mechanical gauges after they quit production of the Brazil VDO versions, and those that have been around here long enough know the nightmares of the later Brazil/China mechanical copies. Henry (former IM) and myself, who developed these gauges with VDO, pushed for good quality mechanical, but had to settle on digital with added logos for warning lights. Anyway, I am a KISS person when I can be, but in this case it was the only alternative and while not infallible, they are the best gauges that have been available for a long time. There are some new clone mechanical gauges coming down the pipeline (not VDO), but unproven so far. Hopefully they get them right and they last. Time will tell.