Got the venerable TP-LINK N750 router still running BB14.07. Criminal negligence, I know... Everything works fine, except for strange WAN disconnects sometimes, so maybe it's time to upgrade. The problem is, I got it set up with root on external (usb) storage and my system skills, as well as spare time are not what they used to be four years ago, and so, I'd like to hear ya'all :
Is it worth upgrading? (obviously yes, huh)
Can I do luci/sysupgrde while on external root or will it fall apart?
Would it be better just to reset to stock, install 19.x and configure the external root again?
Can someone maybe provide me with step-by-step guide / important info / crucial warnings?
Yes, I'm running a TL-WDR4300 myself - there is no reason not to use the current version.
You can, but any kind of sysupgrade will disable your extroot setup - you will have to redo the extroot setup from scratch (and you really shouldn't attempt to keep settings either, as they've changed in all those years and would soft-brick your device; make a backup and only use it for manual reference (don't try to restore it!)).
Nothing special, no device specific quirks to take care of - just do a clean sysupgrade without keeping settings and start up fresh again.