hi there , i am a self thought , cs degree and bootcamp who got the job recently. In general bootcamp are fine and as you say learning the basics of html/css/js before applying to one is a good idea. I did that before joining mine.
In most cases bootcamp arent worth the cost from what they teach , but it also a case by case basis, i think compared to learning on your own vs learning in bootcamp , learning in bootcamp you cover alot more quickly but are v intense and you can easily fall behind if you dont get to speed.
So knowing the objective of why you join a bootcamp is important , udemy courses are great too but can bring you out of scope if you are not careful.
I joined mine in my case because i had a subsidy , the bootcamp was from an actual software dev company which i can gain connections and learn from existing software engineering and also specialize skills difficult to learn alone test driven development and agile processes.
So long you know what you want than you are fine.
Here in singapore we don’t have many bootcamp so from the bootcamp you mentioned i only know about general assembly. I heard that its decent , but the class size and who teach you varies to expectations. But the main value is the group project you do in a group.
you can also refer to my post how i prepare for a job and got the job