@MVLC you might be interested in reading my two reflections on What is a Catholic Hymn? and A Pastoral Look at the Hymns We Sing: Past & Present — The Parish Hymnody Study

Very briefly, I’ve found it helpful to distinguish carefully what kind of hymn I’m dealing with—particularly the difference between liturgical and devotional hymnody—and then ask how that kind of hymn relates to the specific action of the Mass at that moment.

At the Offertory, the Church is preparing the gifts and uniting the offering of our lives to Christ’s sacrifice. Because this is a moment of preparation rather than proclamation, a quieter, devotional hymn can serve the liturgy well. In that sense, a Marian hymn is often appropriate if it supports prayer and does not displace the sacrificial focus of the altar. Historically, many parishes used Marian hymns here precisely in that supportive, non‑thematic way.

At Communion, the Church envisions either silence or a chant that accompanies the act of receiving Christ. If a hymn is used, it needs to remain clearly Eucharistic in theology and orientation. That said, your instinct about Latin is very sound pastorally: using Latin at Communion (or after Communion), when the congregation is already praying silently, can be especially fitting. A Marian hymn works best here when it functions as thanksgiving or contemplation rather than as a “Communion song” competing for attention.

Your broader approach—English for Entrance and Recessional so the people can sing, and Latin reserved for more contemplative moments—respects both active participation and the Church’s long‑standing understanding that participation is not always vocal.

So I’d say Marian hymns aren’t simply a matter of “where they’re allowed,” but of how they function: they work best when they clearly serve the liturgical action rather than replace or reinterpret it. From what you describe, you’re already planning with very solid liturgical instincts.

I was wondering the same. We have four congregational hymns in English with organ accompaniment - except that nobody sings during Communion, the organ is on its own. The people hadn't yet developed the habit of singing any of them really, so I get the chant group to sing the Entrance, Offertory, and Recessional to encourage them and it seems to be gradually taking effect. For April the hymns are:
Amazing Grace
Let us break bread
Christ be beside me
Sweet Heart of Jesus.

The Gregorian chant group sings Kyriale VIII and for Communion next Sunday it will be the second part of 'O Filii et Filiae', echoing the Gospel story of Doubting Thomas. They wanted to sing 'Regina caeli', which is only 30 seconds long, so I plan us to sing it in the two-minute break between the reading of the parish announcements and the Entrance.

At the Offertory, the Church is preparing the gifts and uniting the offering of our lives to Christ’s sacrifice. Because this is a moment of preparation rather than proclamation, a quieter, devotional hymn can serve the liturgy well. In that sense, a Marian hymn is often appropriate if it supports prayer and does not displace the sacrificial focus of the altar
This was mentioned several decades ago in Pastoral Patterns, a publication of J. S. Paluch, related to Monthly/Seasonal Missalette, merged with World Library Publications, and then incorporated into GIA.
I often took advantage of this because of the very small number of hymns the faithful few were able to sing. I tended to use Sing of Mary most often, which they only sort of knew when I arrived. The pastor (Polish) was quite pleased. I only used Marian hymns at the Entrance for the three Marian Solemnities.

At communion, hymns that are directed to Our Lord, or speak of his love and mercy, are acceptable. Jesus the very thought of thee is not, strictly speaking, a Eucharistic hymn, but is very appropriate. Sacred Heart hymns also are fitting.

can give a nice meditative background music for the congregation while they receive.

Sorry, that is NOT what I said, nor what Don stated to eloquently. Marian hymns, yes, at the offertory, but not during communion. Communion needs to be focused on the Eucharist and Our Lord. Our Lady can be honored again after mass.
I don't care so much for the idea of "background music," but sometimes that is all that can be done.

Thanks everyone for your replies, this is helpful and giving me food for thought.
@Don9of11 - that first link is really helpful!
@CGM - I'm actually in Australia where we use quite a lot of English hymns.
@roborgelmeister - am I correct in thinking the Magnificat would be appropriate, since it is giving praise to God rather than honouring Our Lady? I've seen some people say that. I've used Marian hymns for Communion in the past but recently realised that it's rather out of place.

am I correct in thinking the Magnificat would be appropriate,

To quote myself from six months ago:

The Magnificat in its myriad guises and inspired derivatives is the model of a quintessential communion hymn: proclaiming and praising the loving-kindness of God for the gift of receiving his self-gift, and remembering God's many mercies to his people. Many communion antiphons (in the Missal or in the Gradual) take up this register and ambit of theme, even without any express mention of Eucharistic species.