How to scale MySQL transaction wrapped queries?
Read/Write splitting doesn't move all reads to a replica server, it only moves the reads away from the master that are sensible to move... so any reads, for example, done by your processes that are integrating third party data would remain on the master, along with any reads related to transactions, but many other reads, such as report generation or compiling of aggregate/summary views could probably be offloaded to another machine.  

How much this would help vs. how complicated it is to implement is very situation-specific.  The 85%/15% split you're observing seems promising, but it doesn't tell us how many of the reads are directly related to the writes and thus need to stick with the master server.

Another concern with read/write splitting is the read-after-write consistency seen by your users.  If I'm browsing my bank account transactions, that information could easily be read from a replica; on the other hand, if I just did a transfer between accounts, then go back to look at my transactions, the safest thing for the system to do is to read my transactions from the master to ensure that what I just did is immediately visible to me.  Again, the nature of your application is the driving factor in determining the implications of this.

Neither of these points should be taken as advice "against" read/write splitting.  You almost certainly should do some of it... the questions are "how much can you safely do?" followed by "how much will that help?"

When you are integrating the third-party data, something you want to keep in mind is that ideally, database transactions should generally be as quick as possible and as small as possible. From what you've described about using transactions to ensure consistency in the event of a lost connection from a third-party data source, it sounds like you might have room for improvement, here.  The best bet -- probably -- is to retrieve the data from the external source, save it locally, verify its integrity, and then proceed with the integration.  

If a large portion of your load is machine-generated, you probably also have options of making those processes less aggressive in their actions, more deterministic in their scheduling, and more cooperative with each other.  Some of those processes, possibly, could be staged on another server, the calculations done, and the final result sent to your master server... but like everything else, it depends on the nature of the processes.