It dawned on me that I made my previous question more complicated than it had to be.
To put it in a hypothetical way, if I wanted to know a demographic feature at the block group level for given year (let's say 2017), what would be the best ACS product/5-year release to use to get my estimate?
As you know, block group data is only available with 5-year estimates. If you want data for a particular year, your best bet might be to take the 5-year range centered on that year, so for 2017, you'd use the 2015-2019 estimates (which, lucky for you, just came out). But of course it's not a point-in-time estimate, and any trend on either side of the middle year might not be symmetrical.I'm not sure what data you're looking for, but another rich data source at the block group level is the Census Longitudinal Employer-Household Dynamics data (LEHD), which is annual data available at the block level (and can be aggregated up to block group or larger). As the name would suggest, it's focused on employment-related data, both on where workers live and where they work, but it does have demographic information about those workers.
Thank you Bernie, much appreciate the advice and new resource! cheers!