The canonical 20th-century response to "cogito ergo sum" is Wittgenstein's "private-language argument."
https://plato.stanford.edu/entries/private-language/When you thought to yourself, "I think therefore I am," in what language did you think it? In English? The English language is an artifact of a community of English speakers. You can't have a language with grammatical rules without a community of speakers to make that language.
Almost nobody in the English-speaking community has direct access to the internals of your mind. The community learns things through consensus, e.g. via the scientific method. We know things in English via a community of English-speaking scientists, journalists, historians, etc. etc. Wittgenstein calls these the "structures of life," the ordinary day-to-day work we do to figure out what's true and false, likely and unlikely.
As you're probably aware, the scientific method has long struggled to find a "mind" in the brain doing the thinking; all we can find are just atoms, molecules, neurons, doing things, having behaviors. We can't find "thoughts" in the atoms. As far as our ordinary day-to-day scientific method is concerned, we can't find a "mind."
But "cogito ergo sum" isn't part of the scientific method. We don't believe "cogito ergo sum" because reproducible experiments have shown it to be true. "Cogito ergo sum" proposes a way of knowing disconnected from the messy structures of life we use in English.
So, perhaps you'd say, "oh, good point, I suppose I didn't think 'cogito ergo sum' in English or Latin or whatever, I thought it in a private language known only to me. From this vantage point, I only have direct knowledge of my own existence and my own perceptions in the present moment (since the past is uncertain), but at least I can have 100% certainty of my own existence in that language."
The problem is, you really can't have a private language, not a language with words (terms) and grammatical rules and logical inferences.
Suppose you assigned a term S to a particular sensation you're having right now. What are the rules of S? What is S and what is not S? Are there any rules for how to use S? How would you know? How would you enforce those rules over time? In a private language, there's no difference between using the term S "correctly" or "incorrectly." There are no rules in a private language; there can't be. Even mathematical proofs are impossible when every term in the proof means anything you want.
Descartes didn't originally write "cogito ergo sum" in Latin. He originally published it in French, "je pense, donc je suis." But in Europe, where Descartes was writing, Latin was the universal language, the one known to all sorts of people across the continent. For Descartes, Latin was the language of empire, the language every civilized person knew because their ancestors were forced to learn it at the point of a sword, the language of absolutes.
Wittgenstein has a famous line, "Whereof one cannot speak, thereof one must be silent." So must we be silent about "cogito ergo sum." "cogito ergo sum" isn't valid in Latin; "je pense, donc je suis" isn't valid in French. It could only be valid in an unspeakable private language, a language with no grammatical rules, no logic, where true and false are indistinguishable. "Cogito ergo sum" could only be valid in an unusable language where everything is meaningless.
Thereof, we must remain silent.