If you fish a lot, you should keep your eyes open for a Furuno DFF1 black box sounder. I should think that you might be able to find a bargain on one from someone who has replaced it with the new CHIRP DFF1-UHD or wants to sell and upgrade. With a good transducer, the DFF1 will give you an excellent sounder picture on the TZT that should blow away the Dragonfly.
Even better would be if you could find a used DFF3 at a reasonable price. Not so likely as they are quite expensive new.
Did you mean a TM260 transducer? I assume the Dragonfly has its own transducer and you would use the TM260/B260 for the Furuno with a black box.
I have an Airmar GH2183 too and it is a very fast and stable source of heading and GPS data. That must be good on an aluminum boat since you can have it outside away from metal that would interfere with the GPS sgnals. Do the readings track between the PG500 and the GH2183? Aluminum should not have any effect on the PG500.
Don't bother with the Furuno hub. You can use any 12V ethernet switch such as the Netgear FS105 you can buy for $20
The only reason to use the Furuno hub is to get the power control features on a NavNet system with multiple displays. You don't need that so save your money.
I would bite the bullet and drill for the cables and use cable pass through glands or grommets to get a much cleaner and more professional looking installation.