Bottom line have fun
This.
Also, just so you know, I took a long break, though not nearly as long as yours. I've come back a long way and now my GP is comparable to a good amount of the people in the top 50 of my Arena shard (I'm at 7.2M, and we have 6.8 to about 8.1 or 8.2, I think). More than that, for several weeks now I've been taking 1st place often, and I've been taking it every day for at least a week. Yes, it's been a long time in coming, but now I'm becoming a regular fixture at the highest possible level of competition, and I've spent $0 since coming back to the game (and only a little before I left). I'm sure that some of the people I'm competing against have spent way more than I have, and more recently, though we don't have any insane krakens or anything, with the highest GP I've seen at 8.2 (there might be higher ones I haven't seen).
So, yeah. You can push your way into the top 200 in the next 3 months to a year, depending on your shard, and after that you should be able to make steady progress until you're breaking top 50. Once you get there, scoring top 10 is possible, but is dependent on whether you want to spend crystals on refreshing your arena attempts and how much time you have in the evenings to fight battles, wait 10 minutes, fight another battle, etc.
If you stay casual, once you hit top 50 that's probably as good as you'll get for a while, until you outlast other people who quit or until you build up a store of gear and/or crystals that allow you to grab a new meta GL while it's still fresh. Once you get that one key squad, your total GP won't hold you back at all.