There are [many](https://chess.stackexchange.com/questions/24061/what-is-the-highest-number-of-possible-forced-checkmates-in-a-legal-position) [possible](https://chess.stackexchange.com/questions/24033/a-position-in-which-checkmate-is-forced-obligatory/24034#24034) 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](https://timkr.home.xs4all.nl/chess2/diary_14.htm)

Additionally, even with its single line of play, I know of [this](https://pdb.dieschwalbe.de/search.jsp) 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](https://pdb.dieschwalbe.de/P1108329) 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"]