I would use the dictionary also. At first it is a lot of work, but later more and more words are put into your dictionary. So it will become more and more intelligent, and will take les and les time to keep it up to date.

Hi
Your post is titled “Make text before en-space bold”, so I guess that your sentence begins with the word that needs to be bold, followed by an en-space and the explanation. In that case:
1 Make a text style BOLD, as you would
2 You need to find ANY LETTER followed by en-space. So go to the GREP-function in your style, and search for [|\u]+~>. Activate the text style.

Explanation: [|\u] means any letter. Adding the + means 1 or more letters. And end with the symbol for en-space ~>.
It will apply the text style on every letter ànd the en-space, and wil stop when a regular space is found.

Also a tip: use a text style that consists only of a magenta coloring (or an other colour that stands out) to try out your grep style. Sometimes there are unexpected effects, which you might not see otherwise.

Good luck.