I want to Upgrade from Nikon APSC camera D5300? But do not know which system to go for?

I have outgrown my d5300 with Sigma 17-50 f2.8 system. I'm thinking to upgrade next year. With many mirrorless systems and micro four third camers. I'm feeling really lost… Which system i shoudl adopt. Size wise micro four third camers are really good but then again fullframe mirrorless camers arent as big as DSLRs used to be.

I can't push my nikon more than 800 ISO… The noise start to show up. So that is something I'm looking to improve with my system… Between canon and Nikon I feel that canon system is like Apple ios and Nikon is like android… Canon system much easier to follow… Example some nikon cameras do not have auto focus motor built in. So when I rented lenses i really had to look out. I do not think I will stick to APSC system… Would go for a fullframe or micro four third.

Any ideas, help, tips?

Since it seems your main concern is noise, related to high-ISO, look at the various sites that compare various cameras. Does a good job showing high-ISO shots. Since you are interested in noise performance, go with a full-frame. Micro 4/3 will be worse than an APSC sized sensor, assuming the same number of pixels and same level of technology.

Personally, I would go with the Nikon D500 since you have a decent F-mount lens. The noise performance on the D500 is pretty good. But, I'm biased since I want a camera that has long battery life since I take trips lasting weeks where no power is available and replacing my lenses would be costly.

Define upgrade to know what you really want to get into. Define your budget to know what you can realistically buy.

IMO, M43 is not an upgrade to APS-C. If your main reason to upgrade is ISO because you are not satisfied with your entry-level camera, go for at least a mid-range camera model, be it a dSLR or a mirrorless camera. Getting an equally low-end mirrorless will not satisfy you. Hint, ISO is just a tad more bearable with lower end mirrorless models.

Full-frame. Definitely. If you can afford it, go for it. Money not a problem? There's medium format!

Brands. Okay. If you're serious about your Canon is iOS and Nikon is Android analogy then sorry I have to say this. You don't know photography.

If you think Canon is easier to use then it's all right. Nikon does seem to be more technical. Each brand has its own fans. As for that missing AF motor with some Nikon lenses? I like it! It drives the price down when you're buying used. You just have to know what works with what and work with it instead of looking for what's not there.

If you'd truly outgrown your present kit, you'd be able to answer this for yourself.

Buy a Nikon D3s and you'll see what a great camera can do with a mediocre lens. "But it's only 12 MP," you say. You think you've outgrown 24MP but you really don't have a clue. The high ISO performance on the D3s will dazzle you.

I don't think there's any upgrades available, you need to buy a new and better camera.

Go for full frame mirrorless. Smaller, lighter and much better at high ISO.

Manufacturers are moving away from the DSLR towards the mirrorless camera. For example, Nikon announced that there would not be a replacement for the D5600.

When going mirrorless, you are correct in that you no longer have to worry about buying a lens with/without an AF motor. Of course, every single brand other than Nikon never put their customers through this annoying process of trying to figure out which of their FIVE lens mounts were compatible.

As it stands, Sony is still king in the mirrorless market. Canon's bodies have left a lot to be desired but they've knocked out of the park with their fantastic lenses. Never been a fan of Nikon for many different reasons - image quality not being one of them.

If you can afford the Nikon D850, it is probably the best full frame DSLR out today.

As for MFT, I have the Olympus OM-D EM-5 II. I have several shots with ISO 6400 and no noise that bothered me. (All digital photos have some noise.) The Mark III has been announced, so you might want to wait for it.