Fellow RTE user here! I also came to Joplin from OneNote. Please don't give up yet, Joplin (with RTE) is amazing with lots of power that you can tap. However, I will focus on an easy solution for one of your concerns.
To change the font, you will find options under Settings -> Appearance. You can also get more granular control by creating a file called userstyle.css. The userstyle.css can be created in ~/.config/joplin-desktop on MacOS and C:\users.config\joplin-desktop for Windows. Start with something like the CSS below and tweak as necessary to get the results you want. Download, install, and define fonts like "SF Mono Regular" and "Google Sans" for a cleaner look. Remember to restart Joplin after each CSS update.
/* 2024-04-10 */
p {
font-size: 14px;
font-family: "Google Sans", "Segoe UI", Frutiger, "Frutiger Linotype", "Dejavu Sans", "Helvetica Neue", Arial, sans-serif;
/* within a paragraph */
line-height: 1.4em;
/* between a paragraph, but also impacts "fake" lists */
margin-bottom: 1.4em;
}
ul.joplin-checklist {
font-size: 14px;
}
ul li {
font-size: 14px;
margin-top: 0em;
margin-bottom: 0em;
}
ol li {
font-size: 14px;
margin-top: 0em;
margin-bottom: 0em;
}
ul {
font-size: 14px;
list-style-type: square;
margin-top: 0em;
margin-bottom: 0em;
}