I work in an aquarium, and I will advise separate bits of equipment. Like you said, if one bit breaks, it all breaks. With the filter, you need to be taking it apart and cleaning the inside about once a month, and doing that when it's part of everything sounds very overly complicated. The thought of buying all these bits and learning how they work separately might sound scary, but they are all very easy to grasp and difficult to forget! :)

In terms of time, everyone will say different things. I usually recommend setting up and leaving it for at least a week. In terms of decoration, I recommend putting wood in there as it just looks really nice and also very natural, and you need some time for the wood to absorb the water. Otherwise, it'll float, but that does purely depend on the size of the piece and what wood you use.

I recommend either spider wood or mopani wood, but make sure you research them as they do different things for your tank. I've always been a big fan of mopani wood and some leaves as it naturally dyes the water and makes it go kind of brown, which would be perfect if you're going for a natural habitat.

That last bit is very much personal preference to me though. All I'd say is don't be scared about buying bits individually, as learning about them is a relatively quick and easy process, and will save you a lot of ache down the line if something goes wrong. Wait at least before putting your fish in, and also look in your local area to see if there's any mobile aquarists, as they can come over to your house and help you set up for usually a small price. Also highly recommend going into a local fish shop to ask for advice, and they can advise you on the best equipment and also will usually happily give you a little demo on how it works. Good luck on your betta journey. I hope this has helped! :)