Convert Uppercase Become Lowercase?

Hello,

I created a new function for convert uppercase become lowercase in sentence case (the_content). This is the function url >> https://pastebin.com/MLkbeeah

I got a problem with first letter in opening paragraph (< p>) or break (< br>) tag HTML.

This is the sample:

Before:
<p>Lorem IPSUM is simply dummy text. LOREM ipsum is simply dummy text! wHAt is LOREM IPSUM? Hello lorem ipSUM!</p>

Output:
<p>lorem ipsum is simply dummy text. Lorem ipsum is simply dummy text! What is lorem ipsum? Hello lorem ipsum!</p>

My question is:
How to make the first letter in the paragraph remains capital letter?
Or if possible, how to add paragraph and break tag inside preg_split?

Note: I know i can using ‘p:first-letter’ but i can’t, bacause search engine (google bot) still grab lowercase. I think that not good for SEO.

Can someone help me, please!
Thank you.