No apologies needed for the "book". In this situation more information is always better than less. And there's no such thing as too much. You never know which snippet will lead to a good solution. Here are some thoughts in no particular order ... we can sort/focus if you decide something is worth pursuing.
I'm also a Garmin guy. So I always look for a Garmin-based or -compatible solution as first choice. And I have also reached an age where my eyes have become a limitation. - Garmin Nuvi style devices:
You could consider one of the new 6" or 7" devices under the Nuvi, Sumo, Dezl product lines depending upon how robust a unit you need. I switched up to a cheap refurbished low-end 5" recently (Nuvi 2597) just to see if moving to something physically larger would help. It does.
If you combine a larger dedicated Garmin navigation device with Garmin's Mapsource or Basecamp PC planning programs and Google Maps you've got the very best solution. I've tried lots of other things (I'll enumerate a couple later) and always ... Always, end up back here. - Windows tablets:
I've tried a variety of things. You already know that none of them, or nearly so, have built-in GPS receivers so you have to have an external dongle. In the outback that's not necessarily a bad thing because it gives you more mounting flexibility in low signal situations. But it can be problematic to make work with your desired navigation software. And, as you are discovering, it's getting to be nearly impossible to find standard Windows desktop navigation software as all the manufacturers are dropping away.
Most of them do not have Garmin-compatible solutions anyway so let's not waste space on that.
So your only option is nRoute. Yes, it's truly ancient but it still runs quite happily in Windows 8.1 and Windows 10. Hats off to Microsoft for maintaining such solid backwards compatibility in its operating system and to Garmin's original nRoute developers for such long lasting code.
Unfortunately nRoute does not talk to you. So you have to check the screen to follow your route. Otherwise it does everything you want, including maintaining a track log. Here's a link to a review that was done back when nRoute was first split off from Mapsource a decade ago: http://www.gpsinformation.org/penrod/nroute/nroute.html
This solution (nRoute for navigation on a Windows PC) is similar to the first option in that you still use Mapsource or Basecamp and Google Maps for your planning, waypoint and map management, etc. One limitation is that you must use Mapsource-compatible maps (we can discuss that later) for this to work. That's not a huge issue; just something to be mindful of. - External GPS Receiver:
I think the LVC version of the 18x might not have been the best choice. The USB probably would have been preferable since it takes its power off the USB port anyway, and the current draw is minimal: 110mA @ 5vDC for the USB version.
You have clearly figured out how to bodge something that will work but the USB version would be simpler and cleaner. You need to figure how to power the tablet and the GPS receiver from vehicle 12vDC anyway, so whether you do it directly to the GPS receiver or via the USB port is irrelevant.
If you decide to pursue using nRoute the 18x is a good solution because it provides the Garmin proprietary protocol and that's what nRoute wants to see. - Powering a Windows tablet:
This should be simple if you have access to internet ordering. If you do not have a tablet that charges from a standard micro-USB connection there still might be appropriate pre-built solutions out there.
For instance, I have a Microsoft Surface Pro. This has the weirdest power connector ever. But I can go online, search for "Surface Pro car adapter" and find a host of solutions that all allow me to simply plug one end into the cigar lighter of the vehicle and the other has the appropriate connector for my Surface Pro. The prices range from Canadian $9.95 to $84.99 (!).
I'm sorry, but I have an appointment right now and have to run. Later this afternoon I'll do up something on a couple of alternatives. They would not be high on my list but they need to be included.