"And Moshe saw all the craftwork and behold, they performed it as Hashem commanded, so they did, and Moshe blessed them."
And in doing this, Hashem gave a voice to baalei teshuvah (penitents), as our Sages wrote in Avodah Zara 4: Yisrael did not make the [golden] calf for any other reason than to grant a voice to baalei teshuvah....for in the incident of the [golden] calf, they [as a collective] transgressed on those three [the three major sins for which one must die rather than commit: murder, illicit relations, and occult worship]....
....God wanted to grant a voice to the baalei teshuvah [who have transgressed] these three.
Therefore, it says, "And you shall say, 'Return, sons of Man; from here on you have nothing to prevent repentance (teshuvah) because if teshuvah was useful for those three transgressions, then all the more so, it will be effective for transgressions lighter than these.
You can always change for the better.
No matter what you did, Hashem is always ready and even yearning to accept your repentance and bring you back into the fold.
Another thing to remember is that the Sin of the Golden Calf was committed after having witnessed major miracles and after having experienced the closest possible interaction with Hashem.
Yet they still fell off the wagon.
And they still could get back on.
In our generation, I see so many signs of people carrying around toxic shame within them.
And usually, it's not even that they've done anything so bad or unforgivable, it's more that other messed-up people in their life have effectively and wrongly shamed them for basically not being perfect.
Carrying around all that toxic shame turns people harsh, unforgiving, depressed, or bitter, both toward themselves and others. People don't like themselves, don't want to be with themselves, and despair of themselves. In addition, they also turn to all sorts of escapes and distractions.
But the Kli Yakar basically reassures us that look, you probably didn't do anything as bad as everything that occurred during the episode with the Golden Calf. And even if you have committed murder, or engaged in severe immorality or occult worship, you probably haven't done all of these Big Three Sins.
And heck! Even if you have done all three, you can still come back to the right path. And even more so, Hashem Himself wants you to come back and is always willing to lovingly accept the new, improved, penitent you.
And even if you "should have known better" and "could have known better," you certainly could not have "known better" (regardless of how frum your upbringing was) than the generation who sinned with the calf not long after having had an incredible show of God's Existence and His Will, who still sinned terribly - and were STILL forgiven! And then even blessed.
May we all merit to joyfully do complete teshuvah from love.
Although I did borrow a few terms here and there from Rabbi Elihu Levine's translation, this is primarily my own translation and any errors are also mine.
For a wonderful rendering of the Kli Yakar into English, including helpful footnotes, please see Rabbi Elihu Levine's translation.