You’re likely putting too much thought into it. If you want to go with a .htaccess rule, I’d suggest updating the one in pub/ and making a rewrite rule routing to your WordPress directory. Personally, I would consider Magento and WordPress two separate app installations and would add configuration to apache directly for this. When you install Magento with composer, .htaccess files and the like will get clobbered so the separation is nice. Not to mention, having WordPress sitting in a child directory of Magento is a security issue generally.
Check out how to use mod_rewrite, there’s a relevant example right below the fold https://httpd.apache.org/docs/current/mod/mod_rewrite.html