Approach your citations in order of priority. Do the most important 1st, less important 2nd and at some point you are wasting your time and it's time to move on to something more important like reviews, etc...
There is no reason to not do them all at once, and several good reasons to do them all at once. It typically takes about an hour to do a single citation, depending on how counter-intuitive the categories are, if you are pasting description text, uploading photos, etc... Some are fast, and Google Places is an ongoing project.
Create a single, dedicated gmail account specifically for citations and other marketing-related activities, as that account is going to get spammed, and it's best to keep that crap away from a real email account. Use the same complex password for all citations, using one capital letter, one number and one special character, i.e. "Roofer12$". Some citations will not allow a capital letter, while others require it. If case a, drop the capital i.e. "roofer12$". Same thing for the special characters. Use the same PW so that you don't have to maintain an excel doc tracking all the various PW's used for all the various citation sites. Use Chrome when doing this so you can have the PW autosaved and have it available every time you need to log back into the citation site. Create a business logo graphic 200 x 200 pixels, which is the standard for most citations. Use a square image and not a letterbox rectangle, as many citations will clip the sides of the rectangle and it will look shitty and half-assed. Sometimes you will have to resize an image in order for it to look right in the citation. I use "Photo Manager" in MS Office Tools for this. It's clunky and stupidly counter-intuitive, but it's fast and once you master it you can resize a photo in less than 5 minutes. Save a dedicated sub-folder in your "<Business Name> folder named "Citation Images" and save all your images with the dimensions in the title i.e. "roofer200x200" and "roofer1000x1000" or whatever, so that you can easily pick the right size citation photo you need, as soon as you need it.
These bing-bong flips aren't going to deliver the level of service that you want. Fiver delivers exactly what you pay for which is warm dog shit. They make spelling errors and don't know it, and they may or may not give you the login information for all your citations, they may or may not give you good PW's, a good email account, etc... You are hands-on and this is definitely something I would not hand over to barefoot villagers from some 3rd world toilet of a country, like Canada or Norway.
Citations are the single-best bang for your buck, in terms of local rank. I would do the work myself, and not trust someone who's ass and mortgage is not on the line. Others may disagree. They are wrong. Don't be wrong like they are. Except Techxan He's allowed to disagree with me and still have credibility. Even though he will still be wrong. Lol.