Strange code in index.php which blocks my website

[agotel]

Good Evening,
after my website was hacked hacked since Christmas and after I created and maintained it since 2015, and after taking all the appropriate protection measures, I changed the code of the mySQL database, changed the password to url-backend Administrator Joomla , client account and installation of some extensions for protection and security of my website searching on the internet as well as following the advice of the support of the hosting company, I would like you to tell me what I can do to avoid hacking issues and fully protect my website !
I restored the files and the database and the website works normally but after 3-4 days it is hacked again and 1 white blank page appears!
From Plesk and from the server ip when the page is hacked, it opens but displays a message about inappropriate content etc.
Also looking in the logs I noticed that there is a problem with a plugin - extension '' plugin googlemap3.php '' which I uninstalled and if necessary I will replace it with another one!
Also from what I found when the page is hacked usually a script enters index.php and prevents the page from opening!
Removing it makes the page work many times or I need to restore files and database to a previous operating time point.
It is a script that is automatically attached to a porn site.I was able to insert this code that is being added all the time. When I delete it, it reappears.
But this is not a solution to be done every 2-3 days to do a reset, and I have to solve it in another way.
I wasted a lot of time trying to get my website up and running.
I also downloaded the entire httpdocs folder (when my website was up and running) and checked for any viruses, malware, bots, adware etc but nothing was found. ).
I also installed some security extensions (searching on the internet https://www.joomdev.com/how-to-secure-y ... a-website /) and added code to index.php and .htaccess for better security and I changed the permissions of mainly * .php files in the Main Root Folder so that no one can log in and make changes.
From your experience I would like you to look at it "in depth or in detail" and give me a solution for what I need to do to make my website safe and functional.
How can I PROTECT myself so that PHP files, index.php, configuration.php etc are not hacked?
I am attaching screenshots to you according to the above in my email.

Thanks
Yours sincerely
agotel

[gws]

Easy solution go to mysites.guru a security auditing site,the first audit is free. I do not have an affiliation to this site.

[Webdongle]

The things you did not do were many fold. Here is a step by step list viewtopic.php?f=714&t=946026 with links to full explanation.

[agotel]

gws and Webdongle thanks a lot about immediate response and advises.
I will check it.