How do you like the Oasis? I've been using eReaders basically since first gen and still have my old keyboard based kindle. I'm using a Kobo Clara right now that I shifted to for warm lightning because I mainly read at night. The Oasis looks like a really nice unit and I tend to gradually work towards premium tech for things that I frequently use, but it feels hard to justify the cost. At $329 CAD I'm stuck waiting for a decent sale
I absolutely adore it. I come from a different perspective, though. I started with the budget Kindle (no light), worked up to the original smol Oasis, then got this bigger version with the amazing cold/warm backlighting.
At every step of the way, I've had big new, gamechanging features that felt worth that price. It sounds like your Kobo already has the warm lighting, and I've no idea on the screen size.
I will say that the design is absolutely awesome. I love, love, love how thin and light it is, and how the 'bulk' of the device is all shifted to a narrow part on the side, perfect for one-handed, casual reading. The covers, at least on my old Oasis, as I haven't gotten one for this yet, also feel fantastically implemented, and like part of the unit itself.
If you have most of the features already though, from buttons to warm lighting to 300 PPI to waterproofing, it's a little hard to make that recommendation for the price. It is fantastic feeling and looking hardware, though.