He may get the final say, but that doesn’t mean he’s always right, unfortunately. I’ve met many priests who don’t know very much about liturgy or music, who make it up as they go.

True, but it is always wise to remember whose signature is on your pay check.

Lol

For several years in an earlier life I have been responsible for the music in "guitar masses". That was in Germany, though, and there are many decent collections of "new religous song" that are in regular use in the catholic parishes, but these are all in German and thus of no use for you. Beware that you might need to copy sing-along sheets anyway, because some of the "new religuous songs" fitting to the Gospel of the day will be missing in the hymn/song book of your parish.

• The "guitar masses" were the most attended masses of the parish, which was in parts due to the music, but also because they have been prepared by a teen group together with the vicar. Make sure that the songs match the gospel of the day and some other texts that the preparation group has selected.
• For the congregation, it is important to get an idea of the melody. It is thus a good idea to have two melody instruments (flute, recorder, violin, hautbois) that can play a (possibly imitatoric) prelude and accompany the singing with the melody and a harmony voice. Usually, some of the teens will play such instruments, which is a natural way to integrate them, although it can be considerable effort to write arrangements and schedule rehearsals.
• Including stuff from Taizé, as someone already suggested, is indeed a good idea, but this music was written for a different liturgical context, namely the services of the ecumenical cummunity at Taizé. Here in Europe, many teens visit this community and usually they are very enthusiastic about it. It might thus be a worthwhile idea, to prepare "Taizé prayers" with the teens, which include a lot of (vocal) music. Due to the repetitive
  nature of the music, even the four part vocal settings can be learned on the fly and it invites for instrumental improvisations. The Notre-Dame Newman Centre has a good introduction and material.

And for what it's worth, here is a mass setting for "guitar masses" that I have written some years ago, originally for the 110th anniversary of the IKUE to be celebrated in Montreal. That was cancelled, unfortunately, due to the pandemic restrictions, but the setting turned out to be most useful because choir singing was forbidden during the pandemic and it was for only two parts and I therefore sang it quite often with my wife at our parish. The Gloria is to be sung by the choir or soloists (it is in double counterpoint, so it works with equal or mixed voices in any combination):

https://youtu.be/ZNjRaUroyVs

And the Sanctus has a strophic nature with the chorus to be sung by the congregation (optionally in two parts) and the stanzas to be sung by soloists:

https://youtu.be/tSMqnIvTq7o

Due to the many imitatory sections, it might be too complicated, however, for your singers at hand.