I think I can only invest $100 a month for the next year or two, would that be enough?
In my country, $100 per month is already if it's going to be allotted to an investment. Like it's a budget of a single to two person per month for food and other trivial things and that's why it's enough for me if I'll allot that much in the process of DCA.
I am worried about missing big bull market with halving and etf approval.
You'll be much worried if you'll keep waiting for the approval of other people before you invest. And that's why buy now and start your DCA process before the halving or even after the halving. There's more to come to Bitcoin and about ETFs? They'll naturally and likely going to get a higher success rate of approval so, just do your thing and buy.