Someone may have gabc files of English chant ready to go.

But for the future: I recommend learning LaTeX and Gregorio to make the scores, at least. You can insert PDFs as graphics into other programs (although the results may or may not cause issues); at the very least you have more control.

The same is true with your preferred editor for modern notation.

You can also make the whole booklet in LaTeX. I think that it’s worth it, personally. But it’s a lot to learn. What I don’t recommend is using Source and Summit except for quick editing where you need a visual right away but need just the gabc at the end, not the graphic. It isn’t running Gregorio (not an up-to-date version), you have less control, and the graphical export is messy.

Right… I've heard this from people, but the EUOUAE feature of gabc doesn't work (not usually applicable if you print the psalm tone, but maybe you need to set an introit for an experienced schola and wish to print only EUOUAE in that case). And then… just what is going on with everything else. Yikes. (I realize that the vowel alignment is set for English, but it's not any better with Latin.)

You can try out Gregorio if you go to Overleaf.

This blog I wrote about how to plan vespers links to several resources that you may find useful
https://www.antiphonrenewal.com/post/how-to-do-sung-vespers-at-your-parish

@lmassery to the blog post:

Reading: Have a lector read this and end with "The Word of the Lord."

There is no "The Word of the Lord" after any of the readings of the LOTH, in Vespers or elsewhere.