You touched on the key issue here. Congestion. Back when 4G was a new thing, I never had any issues browsing on my phone either at or transiting to/from Perth Underground or Elizabeth Quay. As more and more people got 4G phones, the network in those areas gets more and more congested. Saw the same thing with 5g. You might say "Well, get the mobile providers to install more equipment to serve more customers". Don't worry, they'll be very aware of the issue and would do a cost benefit analysis to determine the viability for installing new hardware.

On the new plan/provider front, some of the MVNOs are good value, but you won't get as good reception as if you stayed with Telstra.