There are some database performance improvements in the upcoming Beta (expected to be very near if not identical to current Experimental) that help the SQL queries run more efficiently on larger databases.
Run PRAGMA optimize upon closing database connection #3745
In terms of DB size (as opposed to performance through wise SQL query plans), improvements exist.
You currently have a –retention-policy option to do progressive version thinning with age, if you want. Some DB sanity verification tests at start of backup are by version. I don’t know how many you have.
I don’t know if all will be enough to avoid splitting, but splitting can also be good for integrity/recovery.