That sounds doable then - if you can get the load balancing aspect to work right it should in principle work.
For the Gdrive, the only extra config parameters I think you will need is
upload_cutoff = 256M
chunk_size = 256M
(my current size for illustration)
This is a pitiful 8MB by default and increasing this very greatly speeds up transfer of large files (with some diminishing returns). I find 64MB adequate, 128MB ideal and 256MB best. Beyond this there isn't a lot to be gained and I'm actually a bit uncertain from looking at my network graphs if that might be some sort of cap as I don't seems to see larger segments transferring above this number (but this not well tested, just a casual observation).
This only affect upload performance mind you, and also keep in mind that each transfer can then eat that amount of memory, so be sure you don't overload the memory and set it reasonable.
Similarly you will want to use large chunks in your cache, as this will do the chunking on the downloads side. If you wanted media streaming and quick opening of streams then you'd actually want to avoid too large chunks, but for pure throughput and if you don't need streaming then the larger the chunks the better for much the same reason as in the uploads. That said, might want to avoid excessive size as serving out files via cache can't happen until the cache has downloaded at least one chunk, so visitors might feel downloads were "sluggish" to open if they were of excessive size. Try to set a size where you can expect the first parts to get into cache within a handful of seconds to get things started. 64MB might be a good place to start but it really depends on the bandwidth you have.
You will probably want to adjust number of transfers as well for rclone in general to handle several concurrent users. This increases the risk of bumping into limits on API requests (1000 requests in 100 seconds) but it should be much less of an issue in serving mostly large files for the most part.
I think that's most of the optimization stuff I can think of for the moment. Feel free to post your config when you have something up and running and me and others here can comment on what you might consider changing at that point. Just make sure to redact your sensitive information (crypt keys and API keys - basically all lines that has those randomly generated codes in them )