Make sure to watch some videos and read up on how to set up Navionics. You mentioned depth. Navionics can be set up to show a topo map, although it won't show "stuff" that is on the bottom, although if you're at speed, you will likely not get that info in time to avoid stuff no matter what you're using. I added Navionics to an old 7" tablet I had before I ever got our boat, just to play around with it. Not plug-n-play intuitive, but not the most difficult thing to figure out either, and there have been a number of nice updates, so at this point, you're going to have a really nice experience with it - IF you put in some time to learn how to dial it in for your use and purposes. My 7" tablet was on its way to an early grave, so I upgraded to a 10" Samsung Tab Galaxy A, and it was perfectly fine for the last 2 years, no overheating issues while using it (and it's in a Unicorn Beetle case, kinda like an Otterbox case). My wife just reminded me that it DID overheat once at the sandbar when we were there for several hours, but after unplugging it and putting it in the shade, it was fine in a few minutes. Maybe get a white case or PlastiDip the thing to a lighter color.
I use Straight Talk for my phone service. My tablets have no data plan whastoever for them, I use my phone as a hotspot and run SOME data through it as needed (upgrading maps, downloading maps, and that's for when I forget to do all of that prior to leaving the dock - it's really not that much at all, prolly a few hundred megs of data used per YEAR). GPS works perfectly fine for ANY phone, regardless of data or connectivity. My iPhone for work has 4 HUGE areas where it will not connect for anything, yet the GPS still works flawlessly. Side note on that, I also always have my personal phone, an Android, and only have connectivity issues in one of those areas, and it's nowhere near as bad as the iPhone. The tablet has been the same experience - waaaaaaaaayyyyy upriver, there's a 15-minute section where nothing gets a connection, yet the GPS works fine.
As for a permanent mount, I have the Ram Mount, I think I've posted my tablet mount around here somewhere too, and there's no issues with it whatsoever. It's a matter of personal preference, but in my opinion, a permanent mount is NOT needed in this day and age, with all the mounts out there. Ram will cost you, as you already are aware of, but it'll work just fine. I picked up a non-Ram mount for the stabilizer bar right next to the pass-through to the front of the boat, but I haven't had a chance to use it yet. I'm hoping it will work just as good, but I've still got the Ram as my fall-back if it doesn't. I know there's less cockpit room in your 19', but it's not THAT much less, mebbe a few inches at most, I think you'll find the Ram mount will be a better and more adjustable option for your needs.
I know that Navionics has options for pulling in info from other stuff (depth gauge, perhaps? I can't remember right now, but I'll see if I can track this info down), so maybe you could get the transducer and use it with a tablet/phone and not have 2 different gauges to mount and pay attention to.
Last thing that came to mind reading some responses here - if you have a 3D printer (or know someone who can print you stuf), or if you're the least bit handy with woodworking tools, you can fashion a hood to go over your item to help with any glare that you might find to be an issue. Cardboard to make a template for your needs, then flesh it out to the finished product.
I went round and round on this subject as well, and I was wondering how the heck to decide on something that wasn't too much for what I wanted to do, making it a waste of money for extra features I'd never use, or something that would require paying a big chunk of change for updates every year or so, or just how to mount some of the stuff that I would have liked. I started with Navionics, and it's served me well enough that I haven't considered anything else. I now have a 10" screen with info that has gotten noticed from more than a few guys wondering what I was running and how much it cost me. My total cost is ridiculously cheap compared to a dedicated unit, but that fits my needs. Nothing wrong with spending the money for something that you feel will fit your needs. My 10" tablet cost me either $150 or $200 from Costco, can't remember, but I know I bought it on sale. I bought the triple suction cup ram mount after returning the single-mount. The single held the 7" tablet just fine, but it wouldn't stand up to the weight of the 10" tablet over time, so I went with the triple - $62 from Amazon at the time.
I suggest you download Navionics and play around with it - see if you can find the "pull the info from this thingamabob right into this app" thing I was talking about, and download a map of your lake to test it out, and again, watch a video and read up on the app to get the best use out of it. There's a lot in that nicely priced app that might serve you well for a few years.
