With DADA2, what happens to reads that don't merge?

You can try running vsearch --fastq_join and then DADA2 in R.

This will not work with Qiime2. Do you have more questions about the two posts explaining why?

If you want to use QIime2 with this data, another option is Deblur.
https://docs.qiime2.org/2024.5/plugins/available/deblur/