If you opt for Bonaire, a couple of things to mention. I assume by willing to accept rustic, you're interested in budget containment, not desperate for a low end rural experience.
Back in 2012 I spent a week on Bonaire based at Dive Hut, a budget option, which I liked. I didn't like it 'better' or 'worse' than Buddy Dive Resort, anymore than I'd like a budget car over a luxury car; it occupied a different niche.
Back around Sept. 2017, I contacted some offerings in Bonaire to get prices for an 8-day all-shore diving solo trip I hoped to do in 2018 (a plan discarded once Aggressor live-aboard fleet put out too good a deal to pass up on another destination I had my eye on). I wanted waterfront (to enable convenient house reef night diving; a house reef substantially increases my dive count, though the resort tends to cost more). Here are my notes from the package deal I liked best:
Prospective Bonaire Trip (Sand Dollar, from Sept. 2017 info.):
1 Studio Condo. ‘ride to the reef’ package for 7 nights is $1579. An extra night with room tax, added $161.50. Upgrading to an automatic truck & add 8th day adds $94. So $1,834.50. On that 8th ‘extra’ day, breakfast adds $16.50 and adding an extra unlimited shore diving day adds $34.
Basic Package: $1579.
Extra night: $161.50.
Automatic Truck with extra day: $94.
—————
$1834.50.
Breakfast 8th day: $16.50.
Extra shore diving day: $34.
$1,885.
Compared to my prior experience with Buddy Dive, that upcharge for an automatic transmission truck is cheaper (I think Buddy charged me $150 once, for 7 days, not 8). I don't know whether you'd get the same package offering for low season; January would be high season, so expect to pay more even if prices
haven't changed.
Airfare is what'll kill you on a Bonaire trip, and from some locations, travel can be lengthly. If you only shore dive, you won't likely be tipping for it, so you save some money (for that tank of gas at the end of the week!).
I'm not pushing Bonaire over Belize (though the water's warm and the weather pretty stable year round). If it's of interest, I thought this info. might help.
Richard.