Where in the manga does it show or state BM's speed is comparable to FTG? I hope you are not using Kakashi mistaking Naruto for Minato one moment as some sort of indication that BM is comparable to FTG because that would be foolish.Firstly, FTG isn't speed, it's instant teleportation.Biju mode can't be comparable to instant teleportation because Biju mode isn't instant.For example, if Minato and Naruto where both on one side of a mountain and were racing to get to the other side, Minato would always win because Minato would instantly appear on the other side of the mountain while BM Naruto would have to physically run over the mountain to get to the other side.As for Minato only offensive jutsu being rasengan, that is not realistic.For one, Kakashi has already stated that Minato created Rasengan to combine with his Nature element.So we know that Minato has at least master one nature element, and going by Kakashi words that most jonins have master at least two nature elements, Minato more than likely has master more than one nature element, completely disproving your "Minato only has rasengan comment.The more appropriate though from you should have been we have only seen one of Minato offensive jutsus.That one would still be wrong since finjutsu can be used for offense, but less wrong then believing Minato only has rasengan.Furthermore, Minato rasengan is more powerful than Naruto's rasengan base solely on the side being much larger than Naruto's rasengan when.
12 years old naruto needed Kurama's chakra to do it though.
I don't exactly see the problem with comparing Kakashi's statement that Naruto's movements were so fast, in BM, that he mistook him for Minato. It speaks volumes on how fast Naruto has become. When you're compared with a teleportation technique. For Kakashi to even make the mistake of thinking it was Minato out there... Yeah, sure. BM is clearly near FTG speed.
And yes, I know FTG is not actual speed but just Minato reverse summoning himself.
And yes, as of now Minato's only offensive ability is the Rasengan. You can assume that he has other jutsu under his belt, but what are they? We don't know. That's not a sound argument at all. Because you can't just say that Minato has other offensive jutsu, probably does, but he hasn't shown any of them. How do you plan on saying Minato harms Naruto or is better/is more powerful than him when Naruto has made more variants to the rasengan than Minato.
Planetary, Rasenshuriken, Giant Rasengan, and doesn't he have some more powerful KCM/BM Rasengan now too?
And I don't think his chakra nature was confirmed. It could be seen that Kishi hinted at that it was wind. But we clearly don't know. And what of Minato's oh so impressive sealing techniques shown to us are capable of taking down a shinobi of similar speed, and this enemy happens to have more advanced forms of the users only known offensive jutsu?
RDS? Doubtful.
The seal he placed on Obito, that rids the summoners control of their summons? The toads would be neutral/aren't controlled by Naruto. Useless.
Minato is certainly not stronger than his son.