I will tell you what I would do, firstly I wouldn't wait too long as the threat of price increases are beginning to come into fruition across the tech world, thanks to the massive price increase in RAM and now storage too. Apple will surely follow suite at some point?
Secondly buy new, if it is a device you want to last I would go new and get that warranty. Maybe buy it from John Lewis for that extra warranty they offer free too. It is where I got mine from. They are not cheap new or second hand so I think new is best personally.
Thirdly, on the screen size that is a dilemma we have all had lol. It really is entirely personal choice, and what you want to use it for. I went from a 9.7" Pro to an M1 11" Pro, and I then upgraded to an M4 11" Pro purely for that OLED screen which is gorgeous. I looked at the Mini and the 13" in person but stuck with the 11". I don't plan on upgrading anytime soon. But I like the 11" as it is easy to hold when it is naked, big enough for what I use it for and is easy to carry round the house. I also place it on a stand when working next to my monitor and it is a good size. I got the Magic Keyboard and for me it transformed the iPad, I use it all the time. Use my iPad for social media, games, content consumption etc. I so on the odd occasion try a bit of animation on it, when I do that I think I a 13" may be better but really I am a complete novice and it isn't that bad.
Mine is glossy too and I have never had any problem with it but I only use my iPad indoors. You can always buy a matte screen protector if you want. It seems the nano glass can scratch more easily?
I use my iPad as my laptop, I have an M3 Max 14 MacBook Pro and it just lives on the desk. I find my iPad much better as a portable device. So this year if I can I want to sell my Mac and get a Studio M5.
Best you can do is write down what you want to use it for, how portable you want it to be, and physically see both the 11" and 13" in person at an Apple Store so you can actually pick them up in the hand. And then decide. There is literally no difference between them bar size, not sure about battery life?
Be aware though the Magic Keyboard does add some weight to the package, so the 11" is lighter in that regard to the 13".
Fourthly, storage and Ram. Hmm tricky one. When I first my M4 I even posted in here as I was debating between the 512GB or the 256GB Cellular model. In the end I decided to get the 512GB, I think I heard it can have faster storage as it uses a faster storage chip or something like that. And I didn't think I would use cellular, I have my iPhone so for the very rare times I would use it I can hotspot. I figured getting bigger stowage would be more useful in the future, so far I am using about 251GB of storage so it was the right choice for me. One where you really need to think I about how much you will use over the next few years. I have some big games on mine, they are the majority of my storage. Biggest one is nearly 25GB
As for RAM it is tricky as more RAM can give slightly increased performance but again depends on what you use it for. In games you can sometimes get better FPS but it isn't a huge jump. I have found 8GB to be more then adequate, I hope it works Ok with Apples future Ai systems but they will make sure it works with all RAM options I'm sure. It is a big price jump though to get the 16GB RAM. If you use heavy photo editing and video editing then more RAM is said to be beneficial. The M chip is ridiculously powerful for a tablet so no worries about that.
Hope that helps if only a little bit? I find my iPad to be an excellent device, use it all the time time.