I think that your last sentence sort of contradicts what you wrote before. I don't remember that resigning Williams was in any way questioned before the season, I'd say we were all pretty happy. Predicting Dawkins decline was also nearly impossible, and I still believe covid is the main reason. I think it is really hard to question FO when it comes to tackles. They kept both above average starters on OL which was working last year, and added 3rd pick in Brown (who seems to be very good) and another developmental depth in Doyle. They also tried to add depth in Hart.
As for guards, they resigned Feliciano, who was serviceable, and added Forrest Lamp who was experienced starter IIRC (I don't remember what went wrong with him). They had apparently some faith in Ford at guard and I struggle to blame them since he played a lot at tackle his rookie season and was injured a lot in the second season. They took a shot with Anderson in draft.
I think people overestimate any GM's ability to upgrade IOL through FA. How many free agents were in fact available who would for sure upgrade our OL? How many of them were affordable? Would you overpay for any of them? I am not saying that it was impossible to do anything, I am just saying that options were extremely limited and you are competing with 31 other teams and have salary cap.
Most glaring (only?) mistake seems to be taking Basham over Humphrey or maybe someone else in draft. I remember that I wanted IOL in the secound round after we took Rousseau and Humphrey seemed like a very obvious choice. But they probably felt that Basham was too big of a value not to take him, and I love this approach in general. Right now it seems that they misjudged their talent, and it also seems that even if they felt that Basham was BPA this was a place to deflect slightly from BPA and fill (very important) position of need. I guess/hope that they will learn from that.
But yeah in general I am a huge fan of building an OL, even at expense of being worse elsewhere. So I hope next season Beane will be more active in this respect (and I am pretty sure he will).
EDIT: Maybe they did not take Humphrey because they saw him as center only. I think he plays center for Chiefs, right? Was there anybody else worth taking in the second? Next OG taken was Wyatt Davis at 86 to Vikings.