I fully understand your issue with the idea that our current players will simply just "get better." To assume that can be a big mistake. Duran got better- Casas and Devers did not. Let's just assume nobody gets better or worse and see what we have...
That being said, I think you are minimizing the overview of our 2024 defense vs our 2023 defense by just talking Yoshida to DH, as if that's the main improvement.
Having Story at SS for a full season is a big plus. Not having Kike at SS for any games makes SS a double plus gain. (It could literally go from worst to first, overnight, with no roster additions.)
Yoshida to DH, in theory, improves more than LF. Not only are Duran and Refsnyder better than Yoshida in LF, keeping Duran out of CF, by proxy, improves our CF D, too. If it ends up being Rafaela, it would be a major gain on D. If it's Duvall or Abreu, it should still be an improvement over Duran in CF. More than half our CF innings, this year went to Duran, Refsnyder, Abreu and Tapia. Add Duvall, and it's about 86% of the innings. (14% to Rafaela and Kike.) If we can get 85% of the CF innings to Rafaela and Abreu, I see a big gain on D at a key "up-the-middle" position.
I'm not sure a full season of Urias and Reyes at 2B will be an improvement on D, but Arroyo led all Sox players in innings at 2B. E Valdezz was 2nd. I can't imagine the two we have now doing worse than they did. (Arroyo + EValdez had 799 ot 1430 innings at 2B. Add Turner, Wong and Bobby D's totals and it's 854/1430 or 60%.) Why can't 2b be a greater improvement on D than LF?
Now, I'm going to go back on my initial promise. I do think Wong can and should improve on defense as he gains experience. It is very common for catchers to improve until at least 30. He was excellent on runner control, but needs to improve on blocking pitches off the mark and maybe how to handle the staff to maximize results.
I will not project Casas will improve. I tried that with Dalbec.
Better at SS, 2B, LF, CF and maybe C with no new roster additions.