You missed the trace lengths on the pico board itself.

[jhinklejr]

You went to a lot of work making all your traces for the inputs the same length but it looks like you did not take into account the trace lengths on the pico board from the chip to the board pins. I designed a similar board but using a Pico2. I opened a schematic editor for the pico2 board and noted each of the trace lengths from chip pin to IO pin. I post them below if you choose to use the information.

Pico PCB Trace Length - mm
mm mil req mil
P0 - 26.2 - 1032 - 672
P1 - 24.0 - 945 - 759
P2 - 19.3 - 760 - 944
P3 - 17.2 - 677 - 1027
P4 - 15.0 - 591 - 1113
P5 - 12.9 - 508 - 1196
P6 - 8.2 - 323 - 1381
P7 - 6.5 - 256 - 1448
P8 - 5.9 - 232 - 1472
P9 - 6.2 - 244 - 1460
P10 - 9.2 - 362 - 1342
P11 - 11.2 - 441 - 1263
P12 - 14.0 - 551 - 1153
P13 - 17.0 - 669 - 1035
P14 - 21.8 - 858 - 846
P15 - 24.4 - 961 - 743

Total Length Match = 1704 mil

[gusmanb]

The board has been matched in order to have no more than 200ps of difference between channels including the pico.