Crackling/popping sound from recorded tape

Hello all.

I have just recently gotten into the cassette hobby.
Had ordered some blank C60 tapes from Recording the Masters.
Got a secondhand Sony TC-FX 170 deck that the seller cleaned, replaced the belts on and demagnetized before sending.
... And, likely the source of my misery, the cheap-ish chinese USB to MP3 walkman player...

It powered on, fast forwarded, played and rewinds just fine! Sound input and VU meters worked, as did all the knobs.

So I popped in my empty cassette, Side B forward (on accident) and recorded my Side A file to the tape, making sure to add 7 seconds of silence to the file so it wouldn't write to the leader tape on accident and mess something up. So I undid the record pause after checking the sound levels and I played the file and started rolling the tape from the start, 7 seconds later it comes to the parts of my file with the sound and voila, I am recording. Let it run, popped my tape into the player and it played just fine!

Went off for dinner.

When I came back, I popped it into the deck again, this time with side A forward properly, redid my side A and then flipped it to do the same procedure for Side B again, but with the correct file, overwriting the side A I accidentally wrote to it.

Now, when playing my Side A, I noticed some crackling/popping sounds at the start of the tape that slowly went away. It only played in one ear, enjoyed the rest of the tape when it fixed itself, then used the auto-reverse feature to play Side B. And near the end of the tape, where the cracking happened on Side A, it started again, in the other ear.

When re-listening side A, the cracking happened further into the tape and with some checks, like flashing a light at it, I saw that at a part, for example, the light's reflection was disrupted by a line going along the short width of the tape, perfectly straight, where the light looked a little mirrored.

What messed up my tape? Is it fixable? Should I toss the cheap player and would a FIIO CP13 or We Are Rewind player fix my issue?

Sorry if this is a lot, I am just worried I am going to be ruining my brand new tapes even more 😞