IIRC, EoR 1+2 are fairly long, Shimousa is fairly hard (harder than LB1+2, IMO) and Salem has lots of text to read.
If all of FGO was an anime, EoR would definitely be considered filler. It’s not bad and a very small amount of scenes have some main plot relevance, but not to a degree worth skipping an event over or requiring to read EoR before the LBs.
If the OP wants to participate in Case Files, they have to prioritize the LBs, especially if they want to get as much out of the GUDA rerun as possible on top of that as well.
Also, LB2 in particular is fairly short, so it wouldn’t be that much of a rush.
And since the OP has over 800 days in the game already, I’m not so sure if their line-up is so weak that they need to “take their time building up their party” anymore. Rather, even if they’re F2P, they should be prepared enough by now.