The best firmware will probably always be 3.60 moving forward, for homebrew compatability reasons. A 3.60 modded Vita can also be easily setup to report to Sony as the latest firmware version, and one can also easily bypass the minimum firmware requirements for some games with patches. There are no differences between any of the current later firmwares and 3.60 besides attempts by Sony to make it harder to install homebrew, so you aren't missing anything, either. There is therefor no real reason to use any firmware besides 3.60.
On that note, in order to have henkaku stay installed, you need to install Enso (you can get the vpk from here:
https://enso.henkaku.xyz/). Use Molecular Shell or VitaShell's FTP function to transfer the VPK to your vita (VitaShell also lets you mount the Vita as a USB drive, if you want to transfer the VPK that way. If you use FTP on Windows, I recommend WinSCP:
https://winscp.net/eng/index.php). After you install the VPK, run it and follow the onscreen instructions to install it. Henkaku will now be permanantly installed on your device until you say otherwise.
There is also another version of Enso you can find called Enso EX. Do not use Enso EX unless you already have a SD2Vita setup and never plan on using a Vita Memory card ever again. Use regular Enso instead. In fact, nowadays it's probably just better to use Storagemgr or YAMT instead of Enso EX in any circumstance if you want to use an SD2VITA.