I do receive FH pendulum serves (long or short) a lot better because I've faced them a lot more often, and can get a pretty accurate read on them.

Now would like to get some advice how to improve my loop against these kinds of serves. I normally try to stand my ground and try to take the serve off the bounce with a mini-loop as that fits my playing style a lot. So I understand for side-topspin I just have to borrow the topspin, close my bat angle slightly and direct it to an awkward position to him (or should I close my bat more and brush harder?). For side-under, I have to work hard to brush the ball so that it goes above the net haha... I'm not quite sure how to increase my success rates other than facing it more often to get used to it? Or is there some kind of technique that allows me to have a greater margin of error when looping them?

Find someone to serve you similar serves and practice against them.  If you understand table tennis to the degree your posts represent, that will solve your problem pretty quickly.

The one thing when facing serves that go long on your return is to play a standard salute stroke with a more closed paddle but less forward motion over the ball to avoid driving it long.  That imparts more topspin and increases the margin for error.  The faster the ball, the smaller the stroke.

Another thing is to note which side you are contacting the ball first on.  Test whether changing that lets you control the spin better.

I find that short serves are usually slow, so even if I take a step back, I have plenty of time to step in. But long serves are usually always fast, which requires more time to react.

Definitely worth a try :)

Anyone who knows the secrets to deciphering these kind of serves?

When I play people who just have crazy spin serves, I have a bad habit of just touching the ball to try and control the spin; bad idea. Make sure you keep imparting your own spin. Don't worry about their next shot, just get it in play and work yourself into the offensive position on the next shot.

If you're standing back as you say, one easy thing for long underspin serves is to wait and let them fall below the height of the table surface, then spin them up onto the table. Waiting will help the spin start to dissipate and the trajectory of the ball falling will be easier to brush along. Obviously that's not the ideal serve return, but it will get things going.

Yes, I do.  Get the TTEdge app and it will all make sense, he is hitting more towards the tip with slice on the backspin and more across the back of the ball on the sidespin/topspin.  The contact for backspin will be closer to his left hip, and for topspin, it will be closer to his right hip. The angles are different, it is just harder to see if you are not used to the subtle variations in contact.

The TTEdge app basically transformed my ability to read and return serves, before I was almost hopeless, now I am hopeless against great servers but since I have my own nasty serves, the playing field is far more level.

I have a nasty backhand serve myself, especially my topspin version, so this is a serve I know well.  Just focus on hitting the left side of the ball and coming over the ball no matter what happens, over time, you will see the ball slow down when the backspin is heavy, even if it seems to be coming really fast.

Maybe I will post some video sometime.

Deep serves should be punished.  I used to serve mostly deep serves and wondered why so many pros opt to serve short.  Then I met a really good looper who loop killed all of my deep serves during a match.  After that incident, I practiced my short serves extensively.  

IMO, "Step back" is difficult than "Step forward" with your back and your knee still bend down. Are you agree?

So stay 1 step behind before he serve. If he serve long, you will have time to loop, if he serve short on your forehand you can step up for return.

Agree with everyone said keep practice 1 hour per day.