Online meeting and dating is a very common thing - definitely not an issue. If you had never met, that might be, but in your case, definitely not something to worry about.
How to make your application good? read the guideline (both the general one and the region specific if you apply outland), and follow the direction. Start gathering proofs ( even if you have lots, you will have to sort through - quality over quantity!), getting the application organised takes a fair bit of time, and can be a little overwhelming. Decide whether you want to apply inland or outland, then download the application package, and start filling out everything. Follow the checklist, don't "add" anything, don't put up a very fancy presentation (binders, dividers, staples etc ) - just follow the checklist, and make sure EVERYTHING is there - go through the forms, and answer all the questions, make sure you have signed everywhere needed.
Get the police certificate started, depending on where you lived before. leave the medicals for the last minute - as they are only valid for 1 year, you want to do them as close to the sending date as possible.
Put together your application, don't look at it for a few days, then go over it, and try to put yourself in a stranger's shoes: reading it, would it convince a stranger that you guys are a genuine couple? if yes, then great. If not, then add pictures, proofs, family letters etc.
It seems like you have a straightforward application, so there is no reason to worry about being denied. and in general : if CIC has doubts, they will ask for more documents, or give you an opportunity to explain. You won't get denied because you forgot something - it will delay the application, but you will get a chance to explain.
Sweden