"you can not have more than 180 sub days at a time"

[Noxifer]

You can

Buy Crystas for 180 days
Put Sub on automatic and that you pay with Crystas
Win

Well, it's not an issue for me personally, since I use recurring subs anyway, but... can you even buy Crysta with your Steam Wallet? Since I don't have my FFXIV via Steam, I have no clue. If you can, then that's a good solution. If you can't, it doesn't really help OP with their problem ^^;