Sins are forgiven by Taubah (Repentance) alone

Quran 4:31:

If you avoid the major sins which you are forbidden, We will remove from you your lesser sins and admit you to a noble entrance [into Paradise].

The following excerpt is taken from “Maariful Quran” Volume 2 pg. 405-406:

Major Sins are forgiven by Taubah alone

From the verse, we find that the expiation of sins through righteous deeds such as wudu, salah, and the rest, which appear in ahadith,
concerns minor sins. As for major sins, they are not forgiven without
Taubah (Repentance); and for minor sins, the condition is that one
should have made the effort to stay away and remain safe from major
sins. This leaves us with a note of warning – if someone, while staying
involved in major sins, goes on performing his wudu and salah, then
this wudu and salah and other righteous deeds will not at all expiate
for even his minor sins, let alone the major ones. It is a warning that
when a person appears on the fateful Day of Resurrection, carrying
the heavy burden of his major and minor sins, he will find no helping
hand to make his burden any lighter.
It will be recalled that some major sins were mentioned in the
previous verses along with the warning of severe punishment for those
who commit them. It is a peculiar style of the Holy Quran that when it
warns of punishment against a sin, it is generally followed by some
aspect of persuasion towards obedience.
In the present verse too, a particular Divine reward has been
mentioned and people have been persuaded to acquire it by staying
away from major sins, in which case, Allah Almighty will forgive their
minor sins on His own. Thus, cleansed of all sins, major and minor, one could look forward to entering the home of honour and peace, the
garden of Paradise.

The following excerpt is taken from “The Quran with Annotated Interpretation pg. 213 under note 11:

“It is said that no sin for which one has sincerely repented and asked for God’s forgiveness is great, and no sin that has been committed continuously and without repentance is considered to be minor.”

The following excerpt is taken from “The Study Quran” under the commentary of the above verse:

“Al-Rāzī suggests that grave sins may indicate varieties of kufr (disbelief), that is, disbelief in God, the prophets, the Last Day, or the Sacred Law. This would be consistent with v. 48, which states that God forgives all sins except shirk (idolatry; R).”

Moreover,

“Although Sufi commentators agree that the truly great sin is shirk (idolatry), they consider shirk to also mean an excessive attachment to worldly things, and even the assertion of the independent existence of one’s own soul in a way that would obscure pure devotion to and utter reliance upon God. Hence the Sufi saying, sometimes attributed to Rābiʿah al-ʿAdawiyyah (d. 183/801): “Your existence is a sin to which no other sin can be compared!” (K, Qu).”