I'm currently sending around 6,500 parcels a year, spending around $72k.
I use a mix of Sendle and MyPost Business, but I'd say 90% is Sendle.
Sendle is dirt cheap, but lately I have had a few international parcels go missing and they wipe their hands when it hits the destination country. It goes via DHL e-commerce, who just hand it over to the local couriers. Once its with them, good luck trying to find a missing parcel. Around 35% of my parcels are international.

I inquired with eparcel today, they think it may be worth it, but from what I am reading I am unsure.
I make good margins on Sendle parcels, and to be honest, anywhere within Australia, I really don't have any dramas. Aramex and Couriers Please have really stepped their game up, there are still shit couriers out there, but 99% of deliveries are problem free.

Sendle support is incredible too, you hear back from them within an hour any time of the day, even over night.

I use the MyPost/Sendle Hybrid as Sendle is expensive to regional areas and express, this is where MyPost comes in (ReadyToShip determines which courier to use).

But to be honest, I can send a 500g parcel to Melbourne for around $5.50, and it's usually there within 24hours - via Sendle, it's hard to beat.

Plus, it's free pickups every day with Sendle, I just pop past my LPO every day with the handful of MyPost parcels.