You poor thing, that sounds so tough, especially with work. I would really struggle with work at the moment so I have no idea how you are coping (mine is currently regressing due to getting on all 4s or sitting up in her sleep).
If you don't want to go down the CC route, I personally would favour the co-sleeper as a regular sleep location until you have got the sleep sorted. Once he goes into your dark room to go to bed, that's where he stays until morning and aim to always get him to fall asleep in the co-sleeper no matter how long it takes. It also saves you going back and forth between 2 rooms in the night. Other option is you move into his room if there is space - I'm planning to do that with my daughter in the next couple of weeks after guests leave then move back into my own once she is settled.
There will initially be nights where you are up for 2-3 hours in the middle of the night trying to get him to sleep but if you consistently use the same technique and keep him in the co-sleeper, he will start to get the message that there is no alternative but to sleep at night time. Although I didn't get too many tears during sleep training, I did get the long wake ups in the night where she refused to go back to sleep and it is tough
I don't know enough about settling techniques for sleep training an older baby (shush/patting and my presence calms my one during regressions like the one we are going through) but I'm sure someone else will be able to advise you on that. Then set a time (6-7 hours) and do not offer anything other than your settling technique from the time you put him down until that timeframe has passed. Then offer one feed if needed and back to settling technique for any further wake ups.
Once you start sleep training, you will get to know the sounds of their cries so much better and you will know exactly when they need you and when to leave them. If my daughter wakes whinging, me going to her aggravates her whereas she can put herself back to sleep within 3 minutes (I can set a stopwatch to it!). If she wakes happy and chatting, nothing is going to get her back to sleep and she is ready to play. If she is crying, I go to her straight away and she calms and goes back to sleep.
It has to get worse before it gets better and I can only imagine how hard that will be for you given you are back at work but it is worth riding out the difficult nights for the end result. Good luck 