LFO volume advice please

You can draw automation with a sine or triangle wave but I don't think you can do it with velocity. I also wouldn't know how to do it over thirty bars. I would say if one arc is over thirty bars, you probably don't have to go up and down again very often so just draw it.

If you just want a volume swell/fade, and not alter at the same time any other functions that the synth may have assigned to velocity, then Melda Production's MMultibandTremolo may do what you want. It's an audio effect, not MIDI so would go on the synth's audio track after the synth.
 
It can handle incredibly long modulation times and just about any wave form you can think of. What's more, it's free.
 
https://www.meldaproduction.com/MMultiBandTremolo?kw=mod
 
Velocity changes would give a different result, as (depending on the synth's amplitude envelope of course) notes would only change in volume as each note was played, while a tremolo will change the volume of notes throughout their length. But for a straightforward volume change over time it's a good effect.