Firstly, what is this cheap flight club you speak of?
Jack's Flight Club (
https://jacksflightclub.com/). I think there's also a US-based cheap flight club called Scott's Cheap Flights or something like that.
We went to Japan for the first time for my birthday and, like you were inclined to do, booked our next trip as soon as we got back!
We almost booked right after we got back as well when we were feeling depressed about being home haha. We didn't only because the cheaper deals at the time were too close to the time we were having my parents visit from the US and then my sister is coming this week as well. Then there were flights during sakura season but, as I imagine you know, prices and crowds are at peak so we decided to skip. When the October/November flights came up we were like ummYES.
Our first trip was 2.5 weeks and we went to Tokyo, Kyoto, Osaka, Kyosan, Takayama and Kanazawa. Since then, we have been to Tokyo twice more, did a driving tour of the whole of Kyushu and went last winter during winter festival time to Hakodate, Sapporo and Asahiyama.
Ah that's amazing! covered a lot of ground it seems! we are only able to do 2 weeks at a time because we have a dog we need my in-laws to watch for us and it never feels like it's enough time haha. We did 16 days (2 weeks + 2 days each end for travel) and this time we're doing 2 weeks including travel and we already feel like we've lost so much time haha. We're renting a car for a day or 2 (husband's request) this time to drive in Hakone, but that's because he's a car nut. I do love a good road trip but we end up just using public transport because it's so good and we just want to get to each place immediately so we don't lose time.
We do actually have flights booked for the Olympics and managed to get some tickets in the US lottery, though I have no idea if we can get a hotel or anywhere to sleep. It is going to be nerve wrecking to find something. Worst case scenario is that we get a refund on our flights (we managed to book with air miles) and forfeit our tickets. We are also planning on going back next February but are still making plans. I REALLY want to go back to Takayama and Kanazawa since we spent so little time there the first trip so I think we might try to concentrate over that way.
I'm not a massive olympics fan normally anyways (we don't really watch the olympics, for example, unless it's on at somebody else's house) so we just knew we weren't even going to TRY to go for the Olympics. I totally get why some people want to do it, but we just don't care enough normally to warrant higher prices and big crowds (their crowds are already insane in some areas that I can't imagine adding even MORE people! That's my nightmare! hah).
Like you, I cannot stop recommending it to people. I love it so, so much. We have convinced several of DHs colleagues to visit there! I am actually helping a colleague plan a trip there now! 
I would love the opportunity to live there and I am actually trying to get into a university course in Tokyo, though it's a total long shot.
Yes we have totally convinced others to go as well and I've just become a mod on Reddit.com/r/japantravel as I love talking about it and giving people advice. Always willing to share my experience and recommendations with people so totally the same as you. My husband and I both want to live there for a bit as well to experience life there, but one thing we are a bit nervous about is work/life balance. Now that I finally have my UK citizenship (ceremony next week), we can realistically have that as an option in the future if we want it without having to go back through all the UK immigration woes again. We're just closing on a house so we wouldn't do it now, but I could definitely see it as a strong option in the future for us. I definitely wish you luck with the application to the uni course!
I am super envious you are going to Hiroshima. We considered it on our first trip and it just stretched us too much. And we tried to fit it in again when we went to Kyushu but it was again too much of a stretch. I also really want to go during autumn and see the leaves change in the mountains. I just think the countryside is so stunning.
We figure we'll land around noon and then we'll make moves to go to Hiroshima ASAP on the Shinkansen so that way we can more or less waste the time travelling on our jet lag day. If we can get there before it's too late, we can walk around the area see a few things but means we can start early-ish the next day before heading to Osaka. Agree that it kind of stretches you thin when you're on a limited period for travel but I think it'll fit in okay with what we're planning this time around (we're not hitting as many places as you hit this time around so it shouldn't be too bad).Will definitely let you know how it is. Tough planning at the moment as I don't think we're fully considering sunset time at this time of year compared to the last time we were there and we could fit SO much in because of all the daylight lol. The country overall is just absolutely gorgeous. I've never seen a country mix tradition and culture so flawlessly with modern society and technology and their attention to detail is impeccable. There's also just so much nature and everybody is very hospitable. I love it.
I'm sure everybody else is bored stiff with our Japan talk but I could talk about it for ages! hahaha
