You could try more diols. If the number of carbon atoms versus hydroxyls matters to bridge with isobutanol, then ethylene glycol is not an indication for what other diols would do.
The very approximative guesswork is that the four carbons in isobutanol are lipophils (possibly a reason to work against viruses), not hydrophilic, and this makes the mix with water unstable. To bridge isobutanol and water, I'd first seek small alcohols whose proportion of C versus OH is between water and isobutanol.
Ethanol is badly available
Isopropanol is badly available
Butanol has already as many C per OH as isobutanol, so go on with diols
Ethylene glycol has few C per OH, it will link with itself rather than with isobutanol
1,2 and 1,3 propylene glycol maybe
1,2 and 1,3 and 1,4 butanediol maybe - I'd favour the asymmetric ones
and so on, choosing for toxicity and availability, stopping well before octanediol.
If Merck or an equivalent still delivers where you live, you could order a dozen compounds and try in parallel.
Did you try if glycerine mixes with isobutanol?