The video is consistent with your earlier statement that you serve that "low" backspin serve, because it works so well in games. If i was to guess i would say you probably create around 30 rps of backspin with that serve. if you are training the 3rd ball attack like in the past video where the other guy just lifted the ball long back to you then there is probably at max 20 rps of backspin in the ball which makes it very easy to power loop for a winner. If it is high enough you could even blast it past the opponent even if you did not know how to properly create topspin.
now when doing the exercise with this current training partner we see the big difference. he does push properly giving his own backspin to the equation making the ball returned having heavier spin than you give him. this is why you quite often have trouble looping it with your current forehand technique.
this training partner clearly has some technical education because he does not only have feel but also quite stable shots all around.
You having a miserable time trying to attack the 3rd ball is the logical result of the way you like to play your forehand currently. even the "loop" against blocks where at most 50% spin 50% power if not more on the power side. If you struggle to have consistency getting an incoming topspin or blocked ball over the net then the issues are more severe and it is only logical that the nightmare continues if you use such a technique to try to loop a backspin ball.
i think you can even hear it quite well. Some topspin loops vs block have a more muted spinny sound but all faster ones are more slaps with a harsh sound. Sometimes the ball is high enough or the incoming topspin is enough for you to clear the net with the slappy technique, but the consistency is obviously not there.
i have fought until now in my reply not to mention the equipment and i don't know what you are using right now. I would not advise you to switch it so close before the season but i think you might have missed a great opportunity to find something that supports your forehand loop development better during the past season break. If i was you i would simply accept that with the current setup and technique you are only able to slap loop real high or empty medium high balls and not think about changing equipment during the season.
For the next season break i would advise you to find a forehand rubber that basically does not work with your slap loop technique and forces you to get that habit out of your system, so you learn a proper loop against any type of ball, especially heavy to mid heavy backspin balls. If you fail over and over with the slap technique i hope it will unlearn to use this technique as first response to incoming push balls.
The other alternative is of course to build your system around your current ability which is what most people do.
good luck.