Edited Nov. 16, 2022 at 9:40 a.m.
For the reasons Hammerwise mentioned, I would say the cheaper of ROR/Toews (if he's available) in terms of cost of acquisition. Then on the back-end barring a Chychrun-trade which I wouldn't be a fan of, I think they go with their current guys, and get a guy who can clear the net like Zub, Gavrikov, or Soucy, again whichever is cheapest to acquire (Schenn's name has been thrown around but I think they would just expect Benn to play instead), and I suspect that they will play over Sandin.
The Leafs would need to move money in, money out so, for both move to happen it probably takes moving Kerfoot+Engvall out (5.75m - possible to keep Engvall if acquisitions can be fit into ~7m) so there would need to be enough retention to accommodate for that. The other pieces that it will likely cost (ignoring 4ths and 5ths for 3rd team retention): is a (1st+4th or two 2nds+3rd)x2 + Robertson + Hirvonen/Voit.
The other approach that they might take if they are confident in the d-core as is:
Add guys like ROR+Barbashev to replace Kerfoot+Engvall and add a Schenn for D-depth.
So, basically a defensive 1/2C to play 3C (preferably with leadership+playoff pedigree like ROR or Toews) + a defensive top 4D/top 6D OR defensive "3C" + top 9 F/middle 6F/"top 6 F" (physical) + defensive top 6D
If they add Chychrun, it should be Sandin+ imo, I think he will want out eventually unless Chychrun is playing his offside), but I'm guessing Dubas will deal with that problem when it happens.
So, again Kerfoot goes out (this trade would be sooner rather than later so perhaps Kerfoot is traded to ARZ then moved at deadline if he can't be traded to a 3rd-team prior). 2023 1st + Robertson + B-prospect + 3rd (if this doesn't get Chychrun, then I don't see him coming to Toronto, perhaps Dubas would give up Niemela but I think he would be reluctant due to Chychrun's injury history). Alternate would be Sandin+Robertson+ 2023 1st + 4th for Chychrun + latest of their 2023 2nds, or Sandin+Robertson+2024 2nd + 2023 4th for Chychrun.
They should then have 2.4m to add a forward or 4.65m if they get rid of Engvall. If Chychrun fits very well and helps transform the Leafs defense, then perhaps they would make another splash on a guy like ROR/Toews, but I think they would just add someone that can fit in the 2.4m left.