I dont think the OTA is in need yet of major cleaning, The spots are really closer to the imaging sensor mainly. So if you are swapping gear around expect them to keep building and cleaning eacjh time is your only solution. If you leave all your gear attached at all times, give them a solid clean and put back in place and leave them alone. Typically a sensor blower bulb is all you need, to gently remove dust particles. Turn the exposed optics or sensor on its side to blow, or better upside down just make sure the blower tip doesn't touch anything. Physical contact cleaning always carries the risk of catching a hard particle and dragging it back and forward scratching sensor or optics. So yes avoid if you can and are uneasy about it. But if you're not swapping gear on and off much you should spend the time to take a good set of flats especially if your gear is always setup in place you can use the flats for all imaging. If you find vignetting in your flats you really should use them always purely for that rather than dust too.
BTW Bintel sell a mild cleaning solution and if you have camera gear and crappy scopes use those to practice cleaning on before tackling you good gear. Avoid cleaning solution alternatives, some may contain additives that can coat and attract dust or even damage protective coatings on glass, just grab the bintel stuff. If your scope needs disassembling to clean you may want to find someone able to do that to help you out, but dont if you really dont have too. My guess is there is only dust on your imaging sensor thats visible in your shots.
Your shots look fairly sharp though and possibly the extra water vapour in the air is slightly hazing things. Also while you've always used 300s subs your camera characteristics will degrade over time so maybe its producing too much noise for your acceptability (imaging is personal and i dont think its a problem, your images show as a quick stretch what I would expect in a single sub anyway.) Theres no obvious vignetting in there either but if you want to make mosaics you really should take flats to help get an even background across joins. But a light pollution filter would probably suit you well. The filter wheel may have gaps allowing dust to get in onto filters, flats taken with each filter should clearly show changing dust features while persistant ones or on optics or sensor. Sensor is most common and people tend to have it open faced up so they can visually inspect it but gravity is carrying particles down into this "bucket" constantly, a fan can cause eddies sucking dust into the sensor too. inspect, but then turn upside down and use puffer to gently blow anything that had just fallen in and keep it facing down maybe cap it. You want the sensor exposed to open air as short a time as possible and try to give gravity a hard time dropping stuff into it. then there's humity and static electricity.
My guess is you have some large dust dirt particles on the sensor that may be large enough to see visually with a moving torch aimed at the sensor. use a bulb blower to shift. Anything that refuses to move try an evaporating cleaning solution, first with a drop at an angle to run over the particle and maybe carry it away on its own or with a qtip or fine artist brush with fluid to dislodge it. With cleaning optics I plan ahead and try to reduce the time they are exposed to air and always facing down as much as possible. reassembling from ota outwards cleaning each step and covering too to reduce airborn particles being carried into the cavity. Slow and careful and gentle as you go and once reassembled try to avoid swapping out parts if you can, my gear is essentially in permanent setup, so only dust on the front element of OTA is of any concern and theres nothing noticeable in my subs like yours and I havent cleaned ota in 2yrs. I spent a weekend or so cleaning the backside, OTA to imager basically, clean, test shoot, examine subs, disassemble, clean, test shoot etc to be confident there are not insects living inside the train or anything obviously causing issue. Technically there will be particles in there that would make a quantifiable difference, but I'm happy with where I'm at with my images. Like I said earlier imaging is often personal where you draw the line in the sand with your efforts and whats an acceptible loss of quality for gains elsewhere.