Your old Garmin is probably still your best bet. Depending on exactly what you really want to do.
I use the crap out of nav while out and about. Last few years, using a tablet. I MUCH prefer a real, dedicated gps unit, but the ones I'd consider worth having for my purposes are just stupid expensive. I like the Garmin chart plotters though, if money were no object. That's what I used for a long time, until it died a few years ago. Same unit you see in a lot of bush planes.
Anyway... Even though it's what I use, I'm not a big fan of tablet for nav, partly because all of the apps suck. You'll have to find the one that sucks the least for you. Gaia could have been the one for me, but it was too buggy and kept locking up so I had to abandon it. Have tried a bunch, don't love any of them. Have pretty much settled on BCN though. It's stable and it works. Raster maps are crap, but that's pretty much the only game in town for nav on a tablet. The Delorme app had great potential, uses vector maps, but a couple of really annoying flaws I just couldn't live with. Can say the same for all the others I have tried which is a few.
On the hardware, just be certain you get built in GPS. Up until recently, that meant iPads were a no-go, but they finally started offering them with built in GPS. Android is a lot cheaper though and if it's mainly for nav, no reason to pay up for an iPad. And the iPad used to have another huge weakness for nav - no external/removable storage. Whatever is built in is all you get. Android you can use cheap SD cards to expand storage. I have a 128GB card in my nav tablet with 120GB of maps on it. That would get expensive in a hurry storing that much map data on an iPad. There are ways to get around it now, but they are clunky compared to just popping an SD card in a droid tablet.
You'll pay up for a good screen with high res, but the best droid tablets actually have better screens than the iPad. Up to you whether that is worth it or not, but it will make a big difference in the nav experience, so I did pay up. Currently running a Samsung Tab S w/BCN. Like I said, don't love it, just the least sucky tablet nav I could come up with for my purposes. Tablet nav is still sucky though. But it works.
- DAA