Sight reading is a stand-alone course. You can take it in combination with any other course(s). It starts out very basic and builds step by step. Getting through Vols 1&2 (Vol 3 on the way soon) is a serious time commitment. It’ll take me over a year, just to get through Vol 1, I expect (but from true beginner status), but each lesson seems to get a little easier as I learn more.
Chord Tones and Scale Essentials could be taken side by side (if desired) as they build on each other in the beginning, then start to branch off after the first 10 lessons or so in each.
I completed Starter Pack, then began Sight Reading, Chord Tones and Scales all at once. I practice some 15 mins to an hour (more 15 mins, unfortunately) most days, but haven’t been disciplined about knocking out lessons every day (this is supposed to be fun, not a grind. I am doing the Players Path in SBL as well as a way of learning songs and think it is great, but it takes time away from TalkingBass. All good, as far as I am concerned.) and am at the end of the first module in each course after 6 months of work. If I was more disciplined I’d probably be twice as far (end of module 2 in each). I’ve got the Walking Bass course waiting in the wings as it will be an application of the Chord Tones and Scales courses, so I am forcing myself to get through those before moving on. Sight Reading is a long-term project!
Good luck and welcome to TalkingBass. We have a great group on the TalkingBass Study Group page on FB and Mark does a live-stream there almost every Saturday at 10:00 Eastern time (USA) where he will answer questions from the group. Worth joining FB for, if just for the Study Group. Best of luck!!