I ended up switching to Optus on my end. I transferred my number about 3 weeks ago with no issues so far. I also went out rural last week and was pleasantly surprised to find Optus now reaches areas it use to not reach when i tested it about 5 years ago. While I am only a few weeks in, I haven't had any overall issues yet. For the sake of testing, I am dual-sim'ing a Vodafone "pay-as-you-go" sim and while I have only made a handful of calls on it, seems to be fine too. If for some reason Telstra steps back up and adds support (and hopefully by them delaying 3G closure they realize support needs to be stepped up and they add Asus support) I might jump back just for the wider coverage.
In terms of budget providers, I don't know how Optus and Vodafone treat their network leaching phone providers so I can't say for certain. The blank sim I ordered is just with Optus and not some smaller brand using Optus towers. I'd say try some blank sims or borrow a sim card from a friend or family that uses the network. All I can really recommend is If you are in the city and don't leave very often, Vodafone is probably the cheapest then Optus but Optus does have better range than Vodafone. In my own research, its hard to actually determine what reach each provider has because some sites that compare coverage have older data. For example, I found alot of sites mention little to no support for Vodafone 5G (some saying it doesn't exist yet) only to find in Sydney there is plenty of 5G support.
I recommend just doing some testing of your own because you still have a month before the closure of 3G telstra support. If borrowing a sim seems fine, order a blank or cheap sim (usually $2 -$12 depending on the provider) and test run it for a bit.
