Will it be hard to get back into this game?

I haven't played consistently since S5, is it now too late because of game changes or is the game largely the same?

Getting back wont be that hard. There are just a few new things such as turret plating and the new bounty system. If you just mussed 3-4 seasons its not that hard.

Just "came back" from a 2 year break or so and they reverted adc item changes, so it's the same as when I left.
You should get used to some item changes and the new runes, which is not as hard as it used to be imo and go through new champion abilities real quick, so that you have an idea bout what their abilities are.
Took me about 2 weeks, which is not that much if you ask me.

Just do a new account and climb, it's probably easier.