I just tested a number of fonts in PTX Print and in LibreOffice Writer. Writer displays small caps in every font I tested. Wikipedia says most word processors ignore “true small caps” glyphs by default, instead they fake them by scaling down normal capital glyphs under the hood. I’m guessing PTXprint does not include this “fake” technique for small caps.
In my PTXPrint Andika New Basic and Gentium Basic do not produce small caps, but Andika and Gentium Plus do. Charis SIL and Charis SIL Compact also both “work” for small caps.
If small caps in Charis does not “work” for you in PTXPrint, I wonder if you might need to update either your PTX Print version or your Charis SIL font version, or maybe both? It’s easy to forget that most SIL software items are regularly updated and improved.
I strongly suspect that Tahoma does not include small caps. Check out Microsoft’s specs for Tahoma, compared with Calibri at docs.microsoft .com/en-us/typography/font-list
Interesting discussion on Wikipedia, if you are interested: Search for “small caps” there.
Finally, if you want a Sans font that looks good you might try Andika. Andika was developed as a literacy font so it defaults to “primer a and g.” But you can change these glyphs in Paratext, PTXPrint, InDesign & LibreOffice to the “normal” non-literacy variants. In the PTXPrint Fonts+Script tab click on the Body Text font. Choose Andika. Then tick Graphite Rendering and click the Select Features button at the bottom of the dialog. For Andika and its variations the feature, “Literacy Alternates” is ON by default. Untick it and click OK in the Select Features dialog. You should now see “litr-0” next to the Select Features button. We use Eng and Glottal variants as well, as you can see in the screenshot.