I recommend the random filter operator by Yaisog.
Install it, and then you can make a simple thing like below to pull a random sentence from any tiddler within your filter condition (tucked into a field, here):
<$let
myfilter={{!!filter}}
focus-tiddler-sentence={{{ [subfilter<myfilter>choose-random[1]get[text]splitregexp[\.\s]choose-random[1]] }}}
>
<<focus-tiddler-sentence>>
Your filter condition could be [tag[Journal]] or something more specific (such as journal entries with a certain further tag, or written more than 2 years ago, etc.)
Of course, this solution expects sentences to be separated by period followed by whitespace. Sentences such as “Mr. and Mrs. I. M. Wright etc. are not invited.” will be mangled. It’s very complicated to get regexp to set aside non-sentence-ending periods. But if you don’t mind that limitation (or want to tackle tweaking the filter), this should get you a coherent unit of text.
Also note this will refresh quite often. If you write the random segment into a temp tiddler on load of the wiki (with a startup action tiddler), that would prevent the random operator from resetting every time you interact with (open/edit) your wiki.