Here are the thoughts of a couple of saints directly on Psalm 115 and an interesting article on St. Louis deMontfort's view of "handmaid"

St. Robert Bellarmine  https://www.ecatholic2000.com/bell/psalms.shtml#_Toc417747255


16–17 The holy soul who offers himself entirely in sacrifice to God, has no pride in him; he rather acknowledges his debt of service, and, agreeable to the command of our Lord, says, “I am an unprofitable servant, I have done that which I ought to do.” “O Lord,” he says, what great thing have I done in paying my vows publicly, in even daring death; in doing so, I only did what I was bound to do; “for I am thy servant,” redeemed from the slavery of the devil by the precious blood of your Son. I am thy servant, not only through your having redeemed me, but also through your having created me; and I am “the son of your handmaid;” that is, I am not simply a purchased slave, because my mother, too, is a slave of yours, by creation as well as by redemption. He calls himself the son of the female slave, not of the male, because no matter how free the father, when the mother was a slave, the child was one too. Hence Sara said to Abraham, “Cast out this bondwoman and her son, for the son of the bondwoman shall not be heir with my son Isaac.” Thus Ishmael was a slave by reason of his mother having been one, though Abraham his father was no slave. “Thou hast broken my bonds.” He tells us he is a servant to the Lord, but that the service is a good one and that he has been rescued from a bad one. As the Lord in the Gospel encourages those who labor, and are heavily laden, to take up his yoke; “for his yoke is sweet, and his burden is light,” he does not absolutely free us from the yoke and the burden, but, instead of a rough yoke, he imposes a sweet one, and substitutes a light for a heavy burden. Thus God completely “broke the bonds” that Satan had bound about us, the bonds of sin and the burden of concupiscence, that weighed us down to the lower regions; in place of which he binds us down by the sweet yoke of his law, and the light burden of his love, through which we are raised and exalted to heaven. “Thou hast broken my bonds;” you have delivered me from a most cruel state of servitude, and wished me to be your servant, your service being, in my mind, a throne. I will, therefore, “sacrifice to you the sacrifice of praise,” and no longer invoke false gods; mammon to wit, the appetite, wealth, and honors, to all of which I was heretofore a slave; but I will constantly “call upon the name of the Lord,” who alone deserves it.


St. Augustine
http://faculty.gordon.edu/hu/bi/ted_hildebrandt/otesources/19-psalms/text/books/augustine-psalms/augustine-psalms.pdf

Who has given unto you to imitate His sufferings, save He who has suffered before for you? And
therefore, "Right dear in the sight of the Lord is the death of His Saints" (ver. 15).
He purchased it by His Blood, which He first shed for the salvation of slaves, that
they might not hesitate to shed their blood for the Lord's Name; which,
nevertheless, would be profitable for their own interests, not for those of the Lord.
11. Let therefore the slave purchased at so great a price confess his condition, and
say, "Behold, O Lord, how that I am Your servant: "I am Your servant, and the
son of Thine handmaid" (ver. 16).…This, therefore, is the son of the heavenly
Jerusalem, which is above, the free mother of us all. Galatians 4:26 And free
indeed from sin she is, but the handmaid of righteousness; to whose sons still
pilgrims it is said, "You have been called unto liberty;" Galatians 5:13 and again
he makes them servants, when he says, "but by love serve one another."…Let
therefore that servant say unto God, Many call themselves martyrs, many Your
servants, because they hold Your Name in various heresies and errors; but since
they are beside Your Church, they are not the children of Your handmaid. But "I
am Your servant, and the son of Thine handmaid." "You have broken my bonds
asunder."

https://www.traditioninaction.org/religious/m057_Slav.htm
 
When the Archangel Gabriel appeared to the Virgin Mary in the Annunciation, she responded, “Ecce ancilla Domini, fiat mihi secundum verbum tuum.” The common English translation is “Behold the handmaid of the Lord, be it done to me according to Thy Word.”

But “handmaid” is a weak translation of the Latin ancilla, which was in fact is the term for female house slave in Ancient Rome. Our Lady presented herself before God as His slave. Thus St. Louis had no qualms or fears to write directly about holy slavery to Jesus through Our Lady.