I thought i'd put my 2cents in, I use Vim + sublime.
Vim is particularly useful for accessing / editing config files on remote machines, because it means you dont have to go through the whole process of sync-ing files after you've pulled it off the remote to edit, just SSH in and edit it locally on the remote itself.
Sublime for pretty much everything else, as i can tweak it how i want it, in fact with the addition of ligatures in the latest dev releases i just bought another license for a friend of mine who's been super jealous of my setup for a while as she's been stuck using dreamweaver (#x-mas present ideas).
Now as to what you should use. To give you all something to argue over im going to make a general sweeping statement upfront right now...
You will never have as 'smooth' an experience out-of-the-box in an open source system as you will with closed/controlled source.
This is demonstrably true, compare the learning curve and/or UX of beginners on linux to mac/win. Granted some distro's do make it easier for the lay person to get started, and most use common features present in win/max DE's to make it easy for converts, but the majority still require some proficiency with tech.
However open source has the benefit of flexibility, provided it's fairly well designed (modular, follows SOLID, etc) you should be able to take the source and easily adapt it to your own needs. You wanna use linux on a Pi? Use it on a Pi or for that matter any IO device, or computers, or clusters of servers, etc.
If you apply this same sort of thinking to your editors...
With IDE's the workflow is easier / very intuitive, but you're counting on the devs to anticipate the use cases and either code it themselves or provide a public API so other people can. As soon as you stray outside that, you're screwed or at the very least you won't be able to solve the problem you've encountered in your editor and most likely you end up hacking on terminal scripts to solve the problem.
The problem is the open source alternative (atom / VScode) are based on a horrible platform (electron). It focuses on cross-platform compatibility but takes a big hit when it comes to performance and so while it's fine for the simple things (discord, slack) when it comes to more performance oriented applications (editors, version control, etc) where algorithmic efficiency becomes crucial you can almost directly see the consequences.
Admittedly the node portion of electron does have C/C++ module capability, however AFAIK it hasn't been exploited in any great degree on the editors in question (or if it has then the perf gains are irrelevant).
Sublime i think is the best halfway point you could ask for. It's closed source enough that the devs can maintain core principles behind the software (performance is one), but the API is comprehensive enough that plugin developers can tweak things as they need. Furthermore it's general purpose / generic enough that you integrate it into a workflow however you want.
As to the downside there's only 1, you have to put the time in to construct your own workflow / custom experience, but it's not like you have to do it every time you open it, and once done you can backup and sync your preferences across multiple machines very easily.
As to the specifics in your question
First, if you're not using SASS (SCSS) or postCSS, start, it'll change your life.
Second, i started using a tiling window manager, since then i've never found the need to have all-in-one monolithic IDE's that have everything including terminals inbuilt. However you might wanna check package control if you like that kind of thing
Third, code completion is quite handy however before that, if you're not using emmet, i would start there
After that you can look at code completion
Also i recommend this to show your colors
And perhaps a styleguide generator, you can go through the following repo and pick out what you want to use along with DocBlockr, i use kss-node