The one and only that really work and don´t produce (pdc) latency is:
Nudge the involved MIDI Items which goes to extern a little bit (ms, smpl or whatever you need) in front and adjust the little Item Snap for offsets so, that you can work with the grid-snap.
REAPER has no Track offsets
I work a lot with extern MIDI and that is also the reason why i am, since 32 years or so
, never ever begin at timeline 1.1.000.
1.1.000-Setup; 2.1.000-Song begin.... so I can always move things in front.
technically the MIDI "-" offsets within the Device Setup can only works up to max -1 Audio Buffersize and + offsets for MIDI to extern is bullshit. Who need + offsets for extern MIDI devices -nobody ^^