First I would like to suggest the Notion that Modalism As sequential is heretical but simultaneous modalism can never be considered heretical as the Trinity states that God is one being in three real modes of existence.
These three hypostasis are the one being of the Father almighty.
The Father’s being is Modalistic.emphasized text
Take for example Thomas Aquinas is concept of the Trinity where it is God’s self love and inner relations within His being.
Yet that the only consciousness of God is the Father’s consciousness, one will, one mind, one authority, one wisdom, one truth, one King of kings one Lord of lords and one great I Am.
This is proven when the Greek text states that the Father is the only true God, meaning that the Father is monos theos.
Monos meaning alone solitary desolate only.
And we can see that it is not Patripassian, as Patripassian’s Believe that there is only one hypostasis but this is unscriptural as we see since the Father is so He lives and because He lives He expresses
Meaning that the Father, Word and Spirit are necessary for the Father’s being or else God is not the living God.
We then see anti-social trinitarianism.
So for the Oneness And the Trinitarian can unite in proper understanding of the Trinity and the Oneness of God in Christ.
If we recognize that any other form of Trinitarian doctrine would be partialism or Tritheism.
However, with Modalistic Monarchianism we see the necessity of the Father’s monarchy but also the distinct hypostasis within and of the Father’s substance/being.
So each mode of existence “hypostasis” is real and distinctness but are distinct personal modes of the Father’s being.
Let me know your thoughts if you want me to expand I can…
God bless be safe.