Firstly, any checkmating problem technically fulfills your requirements, such as this mate in 226. However, based on your examples, I think that you wish my positions in which "forced" means "compelled by the laws of the game," aka the only legal move/s, whether for one side or both.
There are many possible positions in which all mates are forced on the first ply. Since you ask not for that, I shall provide examples with more than one moves\ by both sides.
Here is the known record length for both sides having multiple legal moves, since you said that a single line "isn't too satisfying."
[Title "Alexey Khanyan, Tim Krabbe Website Diary Entry #267 2008, Mate In 11 Moves/22 Plies"]
[FEN "4Q2Q/4r3/4n1n1/1bbK1krn/RR1RR1RR/2qn1R1n/4n1nN/Q3Q3 b - - 0 1"]
[startflipped ""]
1... Ng2f4+ 2. Rfxf4+ N2xf4+ 3. Rgxf4+ Nh3xf4+ 4. Rhxf4+ Ndxf4+ 5. Rxf4+ Nhxf4+ 6. Rxf4+ Ngxf4+ 7. Rxf4+ Nxf4+ 8. Rxf4+ Kxf4+ 9. Qee5+ Qxe5+ 10. Qaxe5+ Rgxe5+ 11. Qxe5+ Rxe5+ 12. Qxe5#
Source: Journal Entry #267
Additionally, even with its single line of play, I know of this absolute masterpiece. It's a checkmating problem in which Black has only one legal move for 39 moves in a row!
[Title "Ljubomir Ugren, Problem (Zagreb) 1976, Mate in 39 Moves"]
[FEN "8/4p3/4p3/4p3/4p3/4pRB1/1P2Prpp/KR1Bkrqb w - - 0 1"]
1. Rf8! Kd2 2. Rd8+ Ke1 3. Rc8 Kd2 4. Rc2+ Ke1 5. Rc7 Kd2 6. Rd7+ Ke1 7. Rxe7 Kd2 8. Rd7+ Ke1 9. Rc7 Kd2 10. Rc2+ Ke1 11. Rc6 Kd2 12. Rd6+ Ke1 13. Rxe6 Kd2 14. Rd6+ Ke1 15. Rc6 Kd2 16. Rc2+ Ke1 17. Rc5 Kd2 18. Rd5+ Ke1 19. Rxe5 Kd2 20. Rd5+ Ke1 21. Rc5 Kd2 22. Rc2+ Ke1 23. Rc4 Kd2 24. Rd4+ Ke1 25. Rxe4 Kd2 26. Rd4+ Ke1 27. Rc4 Kd2 28. Rc2+ Ke1 29. Rc3 Kd2 30. Rd3+ Ke1 31. Rxe3 Kd2 32. Rd3+ Ke1 33. Rf3 Kd2 34. Bf4+ Ke1 35. Bb3+ Kxe2 36. Re3+ Kd2 37. Ree1+ Kd3 38. Rbd1+ Rd2 39. Rxd2#
For one last note, I know of this simple problem that illustrates it with either playing giving mate.
[Title "Eugene B. Cook, The Chess Amateur 1926, Mate In For White And Black"]
[FEN "Bk6/1P6/1P3p2/8/8/2P3p1/6p1/6Kb w - - 0 1"]