I think what you're proposing could also work. And those names you mention are interesting players. Dedic I've been watching since last season and he's a talented young player with a lot of potential but possibly needs another year or two before making a step to a big club imo.
But I don't believe our issue is to do with the fullbacks not overlapping or even underlapping into the opponent's box. I think one of the problems is that when we press high, the opposing goalkeeper has a easy out-ball to the flank which bypasses our high pressure. And the reason for that is due to the lack of aggressive/athletic profiles we have at CB imo. Which means we don't fully commit to the high press as a collective unit because one or both fullbacks at times seem to be instructed to be passive in the press, because otherwise the channel would be compromised due to our lack of pace, power and athleticism at CB. Varane is our best CB in that regard but he misses too many games for my liking and we don't have anyone else who fits that profile.
The reason I think a profile like Todibo makes sense is because he can potentially provide the defensive balance in a team that wants to over-commit in the final third. His defending in the channel is very strong and he's generally very strong at winning ground duels as well as having the ability to progress the ball through the lines. We can still play the same system but the only difference being that Todibo would invert from out wide to form a back 3, when the ball is progressed higher up the pitch.
Todibo is a good fit for the hybrid RCB/RB role and also provides progression in possession. Simikan from Leipzig is another decent player for that hybrid role but he prioritises possession over progression which makes Todibo the more appealing player imo.
Diomande or Antonio Silva for the central centre back role. And if possible, maybe look at Branthwaite, Lukeba and Inacio for the LCB role to rotate with Martinez. Under the Glazer ownership that probably isn't likely in one window. But if the Glazers had any ambition, they'd provide the funds for Todibo in January.