Money is tight and you're content with what you have, so I say stick with it. You can always do better, but I wonder whether an entry-level decent sub won't just make you want a less entry-level one, and suddenly your original budget is out the window. Maybe you're better off not getting on the hedonic treadmill in the first place? Just my two cents.
Benq W1070 projector
Da-Lite High-Contrast (gray) Da-Mat screen
Denon X2000 receiver
Focal 700-series towers and center
JMLab Tantal-series bookshelf rears
Rythmik FV15HP sub