Having a web-browser overlay would be super cool, but I'm not sure if there's enough memory available to the overlay to even be able to render html. It's definitely worthwhile to look into though.
390gb doesn't look right. There should be 2 partitions with one of them reserved for eMMC taking up a fair bit of space. So the other partition that needs be be formatted to FAT32 should be smaller than 390.
TextReaderOverlay is a Tesla Overlay that sits on top of any foreground application that lets you view text files such as text-based game guides from gamefaqs.
Installation
Download and install Tesla Overlay (including nx-ovlloader and Tesla Menu)
Download TextReaderOverlay
Extract the content...
Unless you boot into your NAND, all information is stored on your sdcard, including hekate's boot configurations and eMMC. So if you have two cards, you'll need to set up hekate and eMMC on each of them. There's no shared information stored on the switch itself (other than autoRCM)
I'm using a 400gb formatted to FAT32 for a couple years and haven't had any issue. Avoid exFAT if you don't want your sdcard corrupted sometime down the line.
Version 1.0.1 fixes a bug where input only works in handheld, make sure you've rebooted after dropping the new version in .overlays folder otherwise the old version is still the one loaded in memory