Should I take care about HI-DPI and hi-resolution screens? And how?

basically I like to enhance my existing website. It is an end user portal with many dialogs and also popup windows.

I realized a few times that people with HI-DPI screens struggle with my website (eg Retina Display and other 4K screens). Sometimes, the page is using only about 1/3 of the available place where the sides stay empty. Or it has small font sizes (but not for all?). On hi-resolution displays it sometimes is a huge website and users tend to zoom out a little to get benefit from the screen size. This is an annoying situation.

Sadly, the effort for a complete redesign with some floating layout is to much. The site already uses rem for all dimensions and I can adapt the basic font size of the html element and all element sizes will follow. Meaning, I can zoom the page at whole.

Now the first problem. I read several times, that the default font size of webbrowsers is 16px (Pixel!). What happens on hi-res screens where 16px is VERY small? Do webbrowsers automatically scale all by the factor it needs to get similar sizes? Or should I make my page adapt the size of everything regarding the screen resolution (in pixels)? For example a default font size of 16px on 96DPI and 32px for 192DPI?

I did a web search but I did not find how webbrowsers behave in general. And is it a good idea to scale my page by the screen resolution? Or what is the generic approach of solving this (without a float redesign)?

The second problem is, that I do not have a floating design. So if the screen becomes wider and wider, the layout stretches (eg tables get very wide...). So I currently limit an all surrounding DIV to the maximum size of 1400px to prevent it from looking totally stretched. Not happy with that solution anyway. And not sure how this behaves on hi-dpi or hi-res screens :(

The third problem is, that some tables and lists may use smaller font on hi-res screens to get more advantage out of the fact that it still is good readable. Thus, more data can get shown (eg list of users). But is it a good idea? Should I reduce the font size of such tables for higher screen resolutions?

Any good advice on my questions?