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

[Paladinleeds]

To avoid automatic renewal issues when funds vary. Use Crysta. You can put a set amount in your account and leave it.
If you don't have the fund ready at renew time, then you can just let it lapse. When ready add Crysta and reactivate (instant really).

This has been the method I have been using for quite a while. It is convenient and prevents the auto renews i forget about.

I get round it by using Steam Wallet One-Time-Payments. So then when I'm ready to renew, I renew. Dunno if I'm renewing after my current sub lapses though.