This will be a can of worms, but being the opinionated SOB that I am, I'll weigh in. If you only work alone, try everything and see what works best for you. You can keep using your current hardware and keep 12.4 HD for opening old PT sessions or converting if you get sent PT sessions. There's no requirement for you to move to newer PT versions(hey, the guy next door to me is still running version 8
) and the Digilink license is not required until you move to 12.6(IIRC) and if you decided to move up, its a one-time purchase of $299(not that bad if you make your living with this stuff
).
As for other software,(again, my 2 cents) Pro Tools handles audio editing better than the rest(opinion), but you may find Cubase or Logic much better for heavy midi use(although, 2018 is getting a serious push on the midi side). Sometimes you need to give the other options a good test to make you feel better about PT
In any case, good luck on your next venture!