I too have struggled with advanced formatting in view mode of tmap; I needed standard HTML features - line breaks, bold text, hyperlinks etc. - which proved difficult as {tmap} sanitizes popup labels for HTML code (and a newline does not mean a line break in HTML context; a <br> tag is required).
At the end I ended up using {leaflet} package directly, which on one hand requires maintaining two sets of code for static and dynamic maps + learning two separate sets of syntax rules, but on the other hand gives me the most flexibility in formating.