You're going to want 2' on center minimum for supporting the drywall and then you'll want 5/8 drywall to do the ceiling. OR you could go 4' on center with 2x4's screwed to the bottom of the trusses at 2' on center still using 5/8 drywall. For the time it would take to do the 2x4's I'd just get extra trusses.
I like your 2x8 idea with 2x4's going up to the existing truss. Probably the least expensive way. I'd still go either 16 or 24 on center. I'd get four 2x6's nailing two sets of "L" I'd fasten those to the top of the new joists to act as a spacer and for support. You could put both L's up in place with the new joist location marked on each so as you're putting them up you'd be straight too. The two L's will keep them from sagging while you can then use each L to fasten your 2x4 supports to.
I hope all that makes sense.
Is the room something you're wanting drywall in for looks? If this is just going to be a room where you go to work or play around in a bit, why not just use metal? Then you could go 4' on center and the beauty is, no drywall taping.
There's a chart in the lit below for I joist sizing.
https://lpcorp.com/media/1381/lp-solidstart-i-joists-residential-technical-guide-canada.pdf