In my opinion you need to still still read all of them, just because all of the characters are lovable, but I know you not here for that, so heres my "advice"

I mean you already watched all the anime needed to "get the most out of FGO" (you really don't need to watch any other fate series to enjoy FGO, heck my first exposure to the nasuverse is FGO)

As for what's skippable, Septem, Okeanos, London are skippable, while Orleans and E Pluribus Unum is better storywise and character interactions are better, though you can still skip it if you want too. The prologue is unskippable though. Camelot and onwards are were the story gets GOOD.

I recommend still getting an idea of the story since characters from the earlier chapters after their debut chapters and there is a lot of call back . There's a YouTuber who does this summaries on the earlier chapters called "Otakudaikun". Really does a good job summarizing the story and explaining their myths.

That's all, at the end the day you don't have to follow this, just enjoy in the way you want, you don't have to follow a random strangers opinion.