I think part of the problem is that there are several overlapping roles and different clubs, and different eras within the same club, have cut them up in different ways.
Off the top of my head, the rugby division in a club will worry about a number of things, some of them in conjunction with the commercial division:
1. Facilities and logistics - training facilities, rehab facilities, how to get the players to away games in the best shape
2. Equipment and analysis - video, GPS trackers, analytics around play and training
3. Recruitment and management of coaching staff
4. Recruitment and management of non-coaching support staff - physios, doctors, trainers, analysts etc
5. Recruitment and long term management of players and their contracts
6. Management of the academy player pipeline
7. Scouting of potential recruits
8. Overall talent management and succession planning
9. Coach education
10. Development of the overall playing strategy
11. Development and planning of coaching approach and strategies
12. Day to day interaction with star players
13. Season "block" planning (what's in the Roadmap)
14. Selection
15. Matchday tactics and planning
16. Day to day management of training
17. Team-level match preparation
18. Tactics coaching / playbook
19. Unit-level match preparation (scrum, lineout, maul, ruck, attack, defence, back three)
20. Open play skills and decision-making coaching
21. Setpiece skills and training
22. In game-fitness / pace of play / tactical periodisation
23. Individual player preparation / fitness
24. Rehab and return to play for injured players
From what I can tell, under Deano and COS, the DoR was definitely responsible for 1-13 (COS said that he was the only person who spoke to NEv about his development, for instance) and involved in 14 and 15 with JK. Deano was also a bit more involved in day to day coaching - but both made it clear that Kingston (as Head Coach) was primarily responsible for the day to day coaching and match preparation (and said that his technical analysis and attention to detail about how to play against different teams was exceptionally good).
The Head Coach has mostly dealt with 16-18, and then the specialist coaches deal with everything down to 22 (usually with parallel coaching teams between the senior squad, the academy and possibly the A team when it is playing). A number of the senior players and junior coaches also double up with coaching duties at local semi-pro clubs.
23 and 24 are dealt by the fitness coaches, physios and doctors.
JK said that when he became DoR, he learned a lot about things like player contracts and succession planning that had not been shared with him even as Head Coach, and also that there were a huge number of things that distracted a DoR from the business of coaching and preparing the team. I think it had been OK under Deano and COS because the former had grown up in the DoR role at Tigers as it developed, and the latter has a background in sports administration - but it was challenging for JK as a tracksuit coach.
So Quins decided to split the role into a more hands-on DoR and a GM role, especially with Women's rugby being added to the portfolio. From what I can see, Millard looks after something like 1-7, did 8 jointly with PG, and then PG did 10-17 with 18 being shared with the specialist coaches (and probably with a bit too much input and conservatism from PG).
My best guess at what's now happened is that Millard is looking after 1-10, the triumvirate after 11-22 with some oversight on the lower numbers. The reason I think a senior mentor-coach model could work is that the critical point where we're likely to need additional input and challenge is in the 9-11 space, but that is only a relatively small part of a traditional DoR's time.