1- How can I have bold characters? In Lightroom, I use the Title box to identify these illustrations: https://lucnadeau.ca/fr/oeuvres/4techniques-mixtes/
As you can see on the web page, the titles are not in bold type.
2- In a related issue, how to use the title and caption as keywords to be used by the search module?
In Lightroom:
to your first question, there are font-weight controls for both metadata one and two in the album template (under Thumbnail Grid)… Increase the number to whatever works.
to your second question, image captions are already used in Search, as are album titles.
Image title isn’t included in the search document’s list.
You could add the image title to the Keywords field. There are Lightroom plug-ins that can help speed that up. I think that John Beardsworth’s Search and Replace plugin will do that.
http://www.beardsworth.co.uk/lightroom/search-and-replace/