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Sorry if i missed this in the thread above but I didnt see it asked. Do you have a feather touch type focuser for fine adjustments? A lot of factors mentioned all come into play and many change during a viewing session so it can be a juggling act to chase optimal focus/viewing pleasure. Overhead a target is separated from you by the smallest amount of atmosphere whereas at the horizon is considerably more, so atmospheric thickness and conditions will change greatly during a session. Your scope OTA has its own internal thermal effects coming into play, and while a cool down period stabilises the ota a lot its not a matter of it stabilising to a fixed level where it hold that without change. Local changes in temperature and humidity means the ota can be absorbing and radianting heat energy as well as expanding/contracting too. The amount is minute, but its a factor. Your eye pieces likewise. Then there's your eyes. they constantly adjust to the low light they dont adjust to a set level and hold there, they wax and wan constantly. when you put your eye to the eyepiece and observe a bright target it can weaken your night vision slightly which takes longer to re-adjust again. same with a glance at a phone screen etc.
Everything comes into play even in tiny amounts but they all multiply to a noticable change in viewing quality at the eyepiece. No single answer will solve the problem but some coontribute more than others. I commonly find people think the big tube things is the important part of a telescope but its really the eyepieces that do the hard work with the photons, so people whoe rely on their supplied eyepieces are missing out on the potential of their gear, though in your case you have good eyepieces maybe not the best sizes for your ota. Start from the base up, make sure the mount is stable and firm so it doesn't pick up vibrations from the ground and amplify them to the eyepiece. Check things tat should be tight are tight and things that things that can move are clean and frictionless as you can get them. Periodically during a session inspect your optics for signs of fogging indicating you may need to look at methods to insulate to help prevent condensation. Specks of dust rarely do anything much to viewing so dont be too worried about those but also dont let dust build up either. learn to colimate by sight on a star, many of thoes laser collimators themselves are not collimated and only really get you to a ball park. a 10:1 ratio focuser will greatly help you get to a sharp image for your optics , the optics themselves (refracting lens and reflecting mirrors) are all made differently which is reflected in their pricepoints, along with coatings to reduce internal reflections and loss of photons all come into play. many people think big magnification numbers is all that matters when its pretty much irrelevant completely. Spending the extra dollars up front pays off in viewing pleasure for years to come especially when you are using near the limit of the optics. Every layer of glass or mirror a photon has to encounter to reach your eye reduces its energy (ie a loss) so anything there you can improve or remove can help matters. Avoid looking at your phone or bright things during your session to allow your eyes to adapt as best they can to the darkness. Also be patient and calm at the eyepiece, especially on those fainter targets as the eye will spend time adjusting to that specific view and it will become sharper to you over a few minutes. best to adjust fine focus when you are ready to view to account for atmospheric changes. There's only so much money that can be thrown at the problem and we all want to get closer and sharper with our views but there are practical limits. I don't know how many rings of saturn you should be able to see but the cassini division at least should be crisp to you. its very likely the upper atmosphere where you are is moving fast and causing the watery wobble you see... not much you can do in that case except view when your target transits its highest point in the sky.
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