What would be the most practical solution for me? Does the NEMA 2000
standard work as well as advertised for multi-brand systems? Should I
sell my Lowrance unit and go for a one-brand boat such as Raymarine,
Navman, or Comnav? If I start over from scratch for my primary cockpit
unit, is there any merit in mounting the Lowarnce down below?

NMEA2k does indeed interoperate pretty well. I've got Lowrance fuel flow
senders talking to my Raymarine E-80 chartplotter. I've also got a Lowrance
GPS feeding the E-80.

The only downside to NMEA2k at this point is configuration. It appears you
need to have a display from the same vendor as the sensors in order to get
them configured (or updated). I added a Lowrance LMF-200 display just for
this purpose, but also found it useful to have the fuel data on a separate
gauge anyway. But if I wanted to add the Maretron fuel tank level senders
I'd need to either get a Maretron display or their USB gateway and use a PC.
The config and updating is a one time thing so I'd probably go the PC USB
route.

Most systems these days support both their own networks and NMEA2k. It
appears that using the vendor systems for autopilot and such is the only
solution (today). So perhaps you'd want to start from the needs best served
by the vendor network types and build-out from there. You want the
autopilot or other things you NEED working first and worry about adding on
other stuff once that's sorted.

-Bill Kearney