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First I couldn't access backend of site. When I rename htaccess, i can get into it, but then the problem reappears.

Please remember that the .htaccess file can be modified by other plugins as well. We only manage the code in there's that's between the Admin Tools line markings, if you use the .htaccess Maker feature. It is possible that another plugin is doing something in there which might result in your problem.

Moreover, the .htaccess file is used for something else: telling your server which PHP version you are using. When you change your PHP version your hosting control panel inserts some AddHandler (or, more rarely, SetHandler) lines to set the PHP version handling requests on your site. Since each PHP version has its own configuration it's possible to change the PHP version and mess up your site.

Related to the above, the .htaccess file is also used sometimes to set up the PHP configuration (only applicable when the web server runs PHP as a module instead of through a PHP-FPM server). Changing PHP settings without much care can of course mess up your site.

Originally the sites PHP version was 8.4. I have downgraded to 8.1

Have you, though? You told me that you have to remove your .htaccess for the site to work. In most cases this would mean that now the server is using its default PHP version, not the PHP version you (think you have) selected.

 (BTW when in 8.2 I get a PHP error Your PHP Installation Appears to Be Missing the MySQL Extension.

Every PHP version has its own configuration, including which PHP extensions are installed. When you change the PHP version of your site it's your responsibility to ensure that the new PHP version has the same PHP extensions installed and enabled as your previous PHP version, as well as transferring all configuration options you had with your old PHP version to the new one.

Since PHP 8.2's configuration did not have the MySQL extension enabled, you got an error. You can go into your hosting control panel and enable that extension. If unsure how, ask your host as this is very much host-specific and sometimes even server-specific.

Now when i rename htaccess and get into backend I get this error: You are a spammer, hacker or an otherwise bad person.

That, we can help with. Your IP got blocked. Read https://www.akeeba.com/documentation/troubleshooter/atwpwafissues.html for how to unblock yourself. If you have enabled the administrator secret URL parameter, custom wp-admin URL, or away schedule try disabling them. Before using these features you need to read the documentation to understand what they do, and why they can (as they should!) lock you out of your site when you don't access your login page in the way you're supposed to.

Beyond that and based on everything you told me, it sounds like you may have some issues with your PHP 8.1 configuration. If you keep having issues unless you renamed / remove the .htaccess file I think you should enable WordPress' debug mode, including logging (see WP_DEBUG_LOG on the page I just linked), and take a look at the log entries for any errors. Not warnings, notices, or deprecated entries; just errors. Do they indicate you are missing an extension? You can enable it in your PHP configuration. Or maybe you get a timeout, or run out of memory? You can increase the max_execution_time and memory_limit respectively in your PHP configuration.