To answer your question about big contracts:
2021- Mack, Hicks, Quinn, (Jackson under 10M cap hit, but will be there going forward)
2022- Mack, Jackson. Right now Goldman is scheduled to be over 10M but who knows his status for this year let alone going forward, Smith will be a 9.7M 5th year option so close. Nichols will be close, but should be under 10M at least for next year.
2023- Mack (could be cut), Jackson (could be cut), Smith (re-signed), Nichols (close)
2024- Smith, Nichols?, if Johnson proves to be a CB1 he'll need to get re-signed for 10M+
As for balancing the money spent on offense/defense, I actually don't think you need to make a concerted effort to make sure you're spending 90M on both sides of the ball or whatever. Right now, the only big money on O is Robinson, Graham, and the backup QBs. Dalton's cap hit technically isn't 10M and Foles will never see his 10M next year. Graham will be gone next year, and so may Robinson. Whitehair joins the 10M club next year and if Robinson isn't back, the Bears will likely spend 10M+ on a WR. Other than that, you only really look to spend 10M+ on QBs, LTs, and stud interior guys or an elite RT. Between Fields, Jenkins and hopefully between Mustipher, Borom and possibly a re-signed Daniels you don't need to spend big to have a solid OL (though I wouldn't complain if they did).
Basically, I think the O/D spending will balance out if Fields and Jenkins are as good as we expect them to be. By 2025, the defense will be completely different outside of maybe Roquan and Johnson. Offensively, you hopefully have to pay Jenkins, Kmet, Mooney, and of course Fields. So, I'd rather focus on continuing to add young talent on offense to grow with Fields rather than spending just to spend on non-premier positions.