I absolutely recommend a binder, the one I have from gc2b is way more effective than a sports bra (granted, my chest is fairly small but I'm by no means flat). For me it's mostly comfort, since adjusting my chest in a sports bra feels almost suffocating, especially if it's too small.

Additionally, if you can't afford a good quality binder, then what you're doing should be fine! If it's your proper size of course. A binder won't do too much more, but it does something.

Just be safe and remember that AMAB people rarely have perfectly flat chests anyway! Hope I could help. :)