I guess the wildcard here is what are the Sox still willing to spend to go "all in" in probably a season still (partially?) affected by covid. Of the 4 SP's you mentioned, Kluber seems way too risky, Richards & Paxton are a little pricier, middle of SP cost range, with Richards not reaching 100 innings in 5 years. Quintana is lower end cost, seems a real good fit, and might take a discount with the Sox. Paxton would be a nice signing too if the Sox are willing to go that high. On the other hand, the org strength is pitching so they have to weight that out. Can we trust #4 and/or #5 to Cease, Lopez, Kopech, and Stiever (others)? Perhaps if we had to. Same with pen, if we had to, could we? Perhaps. Successfully? I think for both there's a chance, be it small.
However the biggest org weakness is LH hitting. The 2021 lineup only has 3 LH bats, 2 SH (+ Garcia) and Eaton. The org's others consist of Collins, Sheets, & Yermin. Not much there. Last year, even with the great RH dominated lineup, the Sox batted .254 and were 4 games under .500 against RHP and it may have cost us the division and maybe beyond. This MUST improve! If they decide to go lower end due to other signing(s), my 1st choice would be Daniel Murphy at <=$5. He's a well proven .280-.300 hitter (much better earlier) with decent .OPS. And I think he's still capable of a truly monster year in this lineup.
Sorry but no way with Collins if "all in". His defense is not all that good either though small sample size. Its too important of a position, especially when they probably could get Kurt Suzuki for <=$3.
Its hard for us GM's here not knowing the top end payroll plan for 2021 (& beyond).