He wasn't writing biographies, he was preaching his theology and ethics and settling disputes.
So much rests on what 'we expect' Paul 'should have' written, imo, and not on what he did write (or what was written I should say, since I'm using 'Paul' as a convention to mean 'the writer').
Early Christian writings are not all of the same sort. There are gospels, there are epistles, there are apocalypses and other sorts of prophecies, etc. Only some do bio. The sorts that don't were still being written for hundreds of years, long after the ones with the bio were available (Jobar wants to push the start of the gospels to 130AD but that's not easy to do, especially since the consensus is that they were started around 66AD while the writer of the Pauline Epistles was still alive and were probably circulating orally before that, and in any case, non-bio texts carry on way after 130AD anyway).
One thing to bear in mind is that the writer of the epistles was expecting the end times at any moment. If we don't read them in that light we are probably missing the whole point, imo. His main case for this, the main reason for the supposedly dire emergency was that Jesus supposedly rose from the dead. If he didn't do that, Paul wouldn't have a case. Which is possibly why he bangs on about it so much. Even the bits where he gives advice to the followers can be seen to be telling them what to do to make themselves ready as soon as possible. It wasn't 'general life coaching' and unlike today, they weren't going to have to wait until they were dead. This thing was about to happen. The writer seemed to think, or at least be saying to people, that the new churches weren't even going to be there next month or next year. So whether this or that cult member should or shouldn't get divorced, or be circumcised, was, it seems, in order to be pure and ready in order to qualify for the (very) imminent big event.
I say all this without necessarily having any positive regard for Paul (or Jesus for that matter). 'Paul/Saul' was probably a deluded, slightly well-educated little wanker who wasn't necessarily truthful. He might even have been hitching an ego ride on a recent Jewish cult for his own self-aggrandisement. Why would he want paid or be asking for donations to send back to Jerusalem if he really thought the apocalypse was due any moment? Or he mighta really been a true convert. I dunno.
At some point in time, when the end times didn't come, a lot of Christian teaching gradually switched to 'general life coaching' so that you got into heaven after you died. That was apparently not the case in the mid 50's AD. This is easily overlooked when we read the texts today and perhaps project bits of later or modern christianity onto texts to which they didn't apply to the same extent.
It's clear in the writings. Paul has to reassure some new cult members in Thessalonia because some of their friends and family have died and the end times haven't happened. People's faith is starting to wobble.
He does mention a few things though, including that Jesus had brothers, which is not just found in Galatians, but in 1 Cor. 9:5........but there are few. He gives Jesus's teaching on divorce, the Eucharist, on paying preachers (all in 1 Cor).....
There is more in the epistles about Jesus than many think.
Yes, no miracles, which may mean they were a later embellishment imo.
Paul doesn't say much about himself in the letters neither. Who knows what he said in person.
The epistles say that the mark of a genuine apostle is his or her ability to do miracles and faith healing, by channelling the spirit of Jesus. This could be taken to suggest that Jesus was supposed to have done them.
As to why the Epistles don't cite any supposedly done by the head honcho, they don't cite any done by Paul himself or any of the other apostles either, so it's arguably not really a stand-out omission.
Other than the biggie miracle, obviously. The supposedly cheating death one.
Again, If we look at what Paul does say rather than what he supposedly doesn't, he apparently was talking about a recent Jewish preacher guru bloke who got killed and was buried. That's a 1st citation, and apparently a very early one. Even Richard Carrier and Earl Doherty agree they were probably written starting in the mid 50's. It's way more than we have for any other religious guru from that place and time, and there were probably lots. It was The Messianic Age. The Romans had started ruling Judea in 6AD and the oppressed Jews were looking for a holy rescue to screw over the invading Roman gits*. We read of a few other gurus, including in Josephus (arguably another bit of a shitbag who also switched sides and went over to the enemy) but they have less evidence and it's later. Perhaps we don't question their existence because they aren't invested with any historical and cultural significance for us. Perhaps if we could be more dispassionate (including in some cases not mixing up the issue of 'woo' with 'gurus supposed to have done woo') we might more easily say that we have enough early citations, including away from the epistles and indeed away from christian texts, to at least lean slightly in favour of 'likely existed rather than didn't and probably got the ball rolling for a new cult', which is where I stop, pretty much. There is nothing special here. Just another deluded preacher guru who had a few followers initially.
Also, if you were a Jew back then, making up a Jewish messiah who got crucified would arguably have been stupid and counter-productive (and was apparently the main reason the Jews didn't buy into it) given how humiliating crucifixion was for the Jews. In any case, if you were looking for a stooge, you could have picked one of the Jewish preacher gurus who had their head chopped off in a fight with the Romans. At least that would have been a bit more like the sort of messiah the jews were expecting prior to this one. No wonder the early cult members struggled afterwards to scrape the barrel of their own OT for so-called prophecies for failed messiahs.
* Well, some thought they were gits, others collaborated and enjoyed the privileges that they got out of it. See 'puppet governments and other hangers-on'. These ones probably weren't especially hoping for a messiah. It would be a mistake to say that all Jews were.