Do you think changing the brake oil may sort this?
I did it last year, and feeling changed in better, both for brakes and clutch.
How big is the job for DIY to change the brake fluid?
Easy but long, the process needs a big syringe with a transparent hose.
First you suck all the old fluid inside the reservoir, then you fill the reservoir with the new fluid, then you connect the hose to the brake caliper bleed valve, you slightly open it and you suck the fluid from it WHILE you keep topped the level inside the reservoir. When you see the new fluid coming out from the caliper, you close the valve and change caliper/wheel.
Pay attention to keep the reservoir topped to avoid air entering inside the circuit (an assistant is recommended) and to not wet any part of the car body with the brake fluid, since it's highly corrosive for painted surfaces.
Remember that DOT specifications mean different fluid compositions: DOT3, DOT4, DOT5.1 are glycole based fluids and can be used in the same system (the higher is the DOT, the higher will be the boiling point but shorter the lifespan due to water contamination), DOT5 is a syliconic fluid that MUST NOT be mixed with other brake fluids (even with very small quantities) and it needs specific rubber in hoses, pumps and caliper sealings.