I myself went with the Surface Pro 6, the better equipped 256GB model can be seen in quite a few discount promos, the 128GB i5 model is more commonly discounted.
128GB means slower storage, if that isn't a concern, at least go for the i5 instead of the m3 CPU. I think in terms of bang for buck you're not that far off from a device that will do anything you want in this form factor than can be done comfortably and reasonably.
I have an iPad myself and I was initially even considering going iPad again, but despite the wealth of apps available at the end of the day the Surface can be laptop and tablet and for the tablety things a Surface is MORE than enough, I don't need a selection of 20 different f2p games that all copy each other. Okay that's a snide remark, the iPad IS all about an incomparable tablet app ecosystem, that's undoubtedly the one thing Apple has no doubt going for them. Apple knows, you know it, Microsoft does, Google does...
The question is: if the tablet tasks you need are just reading and OneNote, does an iPad really make sense? Say you're also in the market for a laptop upgrade like I have been. Surface is a no-brainer if all you ever touch on your iPad is YouTube, Prime Video, note-taking app, browser and the like.
Also... the Surface Pen is an absolute godsend and back- and forward-compatible. As is the keyboard. With the iPad you'll be spending money again when Apple says the body length needs to shrink by 4mm. Go figure.
That being said I think all individual factors have to be filled in by you, the only two main pointers I can really give you:
iPads are gonna iPad. That they do really well, but they don't really scrape their hardware's capabilities realistically.
Windows tablets are gonna Windows. They will behave like Windows and if you don't like that, well.... Not good, however:
both of these will get updates and security patches for a long time. Don't bank on super old devices and if you wanna make the purchase last a long while you won't even be looking at the Tab S4 anymore, since it too is due for a refresh soon, the S5e is an obvious hint at a new gen of the flagship tablet launching soon.
In the end your options end up being on the pricier side, but only you can make a call on that. iPads are less pricey indeed, but the new Air gets maximum processing specs with an OS that doesn't really need it and they skimped on areas where you would FEEL the difference a lot more as reader and note-taker like less bezel or a better version of OneNote.