Major problem with accents (and category URLs)

Hello, I’m really getting crazy to find a solution. I really need your help (I’m not a developer).

FACTS/ISSUES

My site has always worked under PHP 7.2/7.3. But Last year, after a WordPress update (to 5.0.x or 5.1.x, don’t remember), I got two issues:

1) [biggest issue] Instead of accents (example: é, í, etc.) and special chars (like “ñ” or “¿”), I had very strange characters instead. The site is in spanish.
2) Problem with permalinks for categories URLs (the word “category” appears into the URLs structure when in Yoast is set to be hidden.
Should be: mysite.com/nameofcategory but it shows mysite.com/category/nameofcategory …. and this link doesn’t work).

I “patched” by downgrading from PHP 7.2 to PHP 7.0 (I know, not the best choice, but I solved). It was a patch, not a solution. And right now I’m still using 7.0 :/

But now it’s time to fix the issue, so I started testing in a staging environment (in order not to “break” my online/visible website).

MY TESTS

I thought the problem was my old template, so I deactivaded my plugins and tested PHP 7.3 with GeneratePress and Twenty-Twenty (default WP theme). Of course, WP is at its latest version.

Results: same issues I had last year (chars problems and incorrect category urls). These templates only works under 7.0, as my current theme do.

Status quo about Characters: After testing & googling I supposed that the issue could be related with ‘DB-CHARSET’ or something similar. FYI these are my data:
– Wp-config:
#define(‘DB_CHARSET’, ‘utf8mb4’);
define(‘DB_COLLATE’, ”);

– PHPMyAdmin dashboard shows:
[General Settings] Collation: utf8mb4_unicode_ci
[Database Server] UTF-8 Unicode (utf8)

I also tried to add “AddDefaultCharset utf-8” to my htaccess but it doesn’t solve anything.

I also tried to reinstall WP (desktop –> updates), but it doesn’t solve anything.

Furthermore, during my tests I discovered that:
*** [IMPORTANT] Looking at the content stored in PHPMyAdmin tables (wp_posts or wp_comments), I can see wrong accents almost everywhere.

*** During my tests under 7.3, everytime I tried to “fix” a post, but once I come back to the editor, the accents back wrong again.

*** I discovered that if I edit an headline (H1) that has accents, this entry doesn’t appears in the backend search (yes, the entry is there, but it shows that the post has “no title”, even if there is content in there).

*** If I edit the short description of my website (general settings, short description) it keep wrong accents even if I try to re-save it.

_________

I have the feeling that problem 1 and 2 can be in some way connected: Database? Server? Miscommunication with the DB? (I don’t really get it, I’m not a developer).

GOAL: Need to fix both issues (accents + category url) under PHP 7.3. But I don’t really know what to do now and whatelse to try now.

Can you help me?

Thank your kind help.