disclaimer: Im not an electrical engineer, rather software engineer with a soldering iron... which is probably a dangerous thing - so take everything I say, here or other threads, on electronics with a huge pinch of salt, and proceed at your own risk.
if you search on the forum you'll find some ideas on many of these topics... i know I've been there.
Ive listed below, some (not all), of the discussions I had, that i found useful, but there are lots more (search eurorack)
for gates/triggers, Id assume a cheaper level converter would do the trick
for others 0...5v I used a rail to rail op amp
if you need audio rate, then yeah your going to need to use the codec outputs... which you only have 2 of.. so worth considering if you can get away with GP outputs instead... perhaps the D will smooth anyway.
to take up to 5v, you will need a 5v supply - ideally from the D, but there is a 5v on the axo board.
for -5v/+5 your going to need a negative supply... on eurorack you would take from its supply, as there is not one on on axo board - fortunately, this was not a problem i needed to handle in my case... but means unfortunately, means i cant help you. ... probably easier if you stick with 0..5v on the lfo if possible.
note: i didn't try to get accurate CV voltage (e.g. for pitch) , if you need this... Im not sure if this has extra requirements on the opamp, and Id expect you need to needs some kind of 'mapping table' on the Axo side to get the 0..64 to accurately map to 1v /octave .