How fit were you, in relative terms, when you got 30mins? If very unfit, then dropping 5 mins off your time is probably just a matter of training properly. If you considered yourself pretty fit when you got 30 mins I suspect 25 mins isn’t possible.
In terms of cheap-ish kit upgrades, a set of 2nd hand tub wheels is probably the best thing you can do, although I am not an expert on time trials (it has been 15 years since I last did one!)