I asked Gemini 3 Pro about the relative difficulty of each project in the list and got the following (parenthesized notes are also by Gemini). Gemini noted that the time estimate is based on the assumption that you already understand the theory (which time estimate would extremely vary anyway) and only accounts for pure PoC implementation and debugging. The numbers look reasonable at my sketchy glance but of course YMMV.

    [Difficulty: Low]
    42. Twitter Trends                        5--10h (If you understand the probabilistic math)
    2. Wordle Solver                          5--10h (Pure logic/algorithm)
    17. BMP Codec                             5--10h
    23. Auth Server (JWT)                     5--10h
    24. Autocomplete System                   5--10h
    66. Browser Extension                     5--15h
    15. Diff Tool                             8--15h (Algorithms heavy)
    9. Six Degrees of Kevin Bacon            10--20h (Classic graph problem)
    7. Googlebot (Crawler)                   10--20h
    65. Make                                 10--20h
    
    [Difficulty: Moderate]
    32. Web Server                           10--20h
    41. Time Sync Daemon (NTP)               10--20h
    53. Malware                              10--20h
    58. Malloc                               10--20h
    63. Shell                                10--20h
    19. Quantum Computer Simulation          15--25h (Assuming you know the linear algebra already)
    26. Background Noise Remover             15--25h (Math/Signal Processing heavy)
    11. Procedural Crosswords                15--25h
    39. CDN Caching                          15--25h
    47. Ray Tracer                           15--25h
    57. Load Balancer                        15--25h
    61. CI System                            15--25h
    62. Random Forest                        15--25h
    67. Stock Trading Bot                    15--25h
    56. Lock-Free Data Structures            15--30h (But debugging is painful)
    16. Visualize Object-Oriented Code       15--30h (Language parsing is the bottleneck)
    5. Container (No Docker)                 15--30h (Requires deep Linux systems knowledge)
    8. DNS Server                            15--30h (Strict RFC compliance required)
    70. OpenGL                               15--30h
    12. Bitcask (KV Store)                   20--30h
    38. Wikipedia Search                     20--30h
    50. Amazon Delivery (Vehicle Routing)    20--30h
    46. Zip                                  20--35h (Algorithms heavy)
    1. Bittorrent Client                     20--40h (Binary parsing and managing async network states)
    18. Filesystem (FUSE)                    20--40h (Debugging kernel interfaces can be slow)
    60. Smart Home                           20--40h (Hardware integration eats time)
    40. TikTok (Feed)                        20--40h (Mostly frontend/UI state complexity)
    21. Redis Clone                          20--40h
    29. Road Network                         20--40h
    31. Evolutionary Design                  20--40h
    34. Git                                  20--40h
    59. Netflix (Streaming)                  20--40h
    69. Automated Journal                    20--40h
    13. Audio Fingerprinting                 25--40h (DSP is sensitive to parameters)
    52. Knowledge Graph                      25--45h
    64. Bitcoin Node                         25--45h
    14. Dangerous Dave (Game)                30--50h
    48. Programming Language                 30--50h
    
    [Difficulty: High]
    33. Depth Estimation                     25--50h (Computer Vision math)
    35. GDB (Debugger)                       30--50h (Low-level systems programming)
    72. Audio Multicast                      30--50h (Syncing audio clocks over network is hard)
    43. SQL Optimizer                        30--50h
    36. Neural Networks                      30--60h (Debugging gradient calculations is tough)
    71. Laser Tag                            30--60h (Hardware debugging)
    3. Deepfake (Optimal Transport)          30--60h (Math-heavy; debugging matrix operations is difficult)
    51. Kafka Broker                         30--60h
    20. VLC (Video Player)                   40--60h (A/V sync drift is very difficult to get right)
    28. Google Maps                          40--60h
    30. Collaborative Editor                 40--70h (CRDTs are conceptually dense)
    37. Chess                                40--70h (Performance optimization is a rabbit hole)
    45. VPN                                  40--70h
    27. Dropbox Clone                        40--80h (Conflict resolution and sync logic are extremely error-prone)
    4. Spreadsheet                           40--80h (Cycle detection and UI state management are tricky)
    10. RAFT                                 40--80h (Distributed systems are notoriously hard to debug due to race conditions)
    68. Browser Engine                       40--80h
    73. Decentralized Internet               40--80h
    49. Messenger                            50--100h
    22. Video Editor (Client-side)           50--80h (Browser constraints + heavy compute)
    
    [Difficulty: Very High]
    44. Anonymous Voting                     40--80h (Cryptography is unforgiving)
    6. Geometric Theorem Proving             50--100h+ (Essentially building a symbolic AI engine)
    55. TCP/IP Stack                         60--100h (TCP state machines are massive)
    25. SQLite Clone                         60--120h (A database engine combines almost every discipline of CS)
    54. Game Boy Advance Emulator            80--150h (Requires extremely precise bit-twiddling and timing)