HI @Dawud922,
A very recent discussion regarding the justConcatenate option you mentioned which I advise against. Assuming you meant 515f/806r primer set, you are right that the overlap with a 2x150 run is not sufficient for proper merging which would explain why dada2 is failing to merge these.
You shouldn't... I would just discard your reverse reads and use the forward reads only moving forward.