I’m not familiar with the components you have, so I can’t really judge if the Node is actually the weakest link. But some Audiogoners and pro reviewers have compared the Node’s performance to much pricier streamers (the TAS review of the Node 2 said it was comparable to streamers with an extra 0 behind the price; and I believe the newer Node is significantly better than the Node 2 in SQ).
If you’re dissatisfied with the BluOS app, that might be a good reason to upgrade; on just a sonic basis, I suspect if you went for a used, high-performing DAC you would be more likely to hear a benefit than upgrading the streamer.
Also, consider upgrading the PSU of the Node; I just got an email from Vinshine Audio about their upgrade: