Help with .htaccess setup

I have static HTML pages in my domain root (lets say https://mysytez.com)
I have moved WordPres install into a subdomain (lets say /subz/)
I also have .htaccess directives to;
(1) Remove .html from static pages
(2) Allow WP URLs to remain as if they were in the domain root ( as instructed here https://codex.www.ads-software.com/Giving_WordPress_Its_Own_Directory)

The setup works great until I need to access WP backend, at that point I start getting redirect errors.
The two set of .htaccess directives below both do what they are supposed to on their own, but I suspect when setup together there is obviously some incompatible redirection taking place.
Unfortunately, I have very little understanding of regex to understand what the directives are doing and how to fix the issue, would someone be able to tell me how to fix this?

I know the easy option would be to install WP in a folder, but I don’t want the folder name showing up in the URL if I can avoid doing so.
Thank you in advance

## Begin Remove extension from html pages ##
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^([^/]+)/$ $1.html
RewriteRule ^([^/]+)/([^/]+)/$ /$1/$2.html
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !(\.[a-zA-Z0-9]{1,5}|/)$
RewriteRule (.*)$ /$1/ [R=301,L]
## End Remove extension from html pages ##

#BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteCond %{HTTP_HOST} ^(www.)?mysytez.com$
RewriteCond %{REQUEST_URI} !^/subz/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /bwms/$1
RewriteCond %{HTTP_HOST} ^(www.)?mysytez.com$
RewriteRule ^(/)?$ subz/index.php [L]
</IfModule>
#END WordPress