"Sustain" is generally in the gift of the guitar! There are some luthiers on the strength? I bet they can suggest some tweaks? Meantime, if it has a bolt on neck, tighten the bolts!
However, how exactly are you running things? If using 'Direct Monitoring' you are stuck with a 'dry' signal. You could use a touch of reverb from the DAW but that supposes the interface/PC system can give low enough latency.
BTW, I do NOT agree that "All interfaces at around $200 are about the same" ? Sweetwater have the KA6 for $199 and that beats any AI I know of at the price for latency, connectivity, driver solidity and bundled/downloadable software. The converters are bloody good as well!
Dave.
Yes, I'm recording the signal completely dry then adding my effects afterwards. I didn't even consider recording it wet, but now it's been mentioned that might just be the key.
Get the sound I want then record in.
I usually record with the output of the guitar going in muted so the latency doesn't throw my timing.