Soft braking that doesn't stop well, what could it be?

Air in the pipes is a pretty likely possibility, especially if the pedal feels spongy and braking isn't as effective. When there's air in the system, the fluid doesn't transmit pressure properly and that's why you feel that soft brake. Even if the level is fine, air could've gotten in through a tiny leak or just because something came loose. The first thing I'd do is check all the connections on the brake hoses really well, especially at the wheel cylinders and the master cylinder. Sometimes a hose has such a small leak you barely see it but it lets air in.

If everything looks good with the connections, then yeah you gotta bleed the system. It's not complicated to do it yourself, you just need a screwdriver, a syringe or siphon, new brake fluid and someone to help you press the pedal while you open the bleed valves on each wheel. You start with the wheel farthest from the master cylinder (usually the rear right on a Clio) and work toward the closest one. Bubbles come out first and then clean fluid, that's what you wanna see. With 11 years welding I'm used to meticulous work, so brake bleeds are pretty chill for me.

Now, if after bleeding the pedal is still soft or you get the same problem a few days later, there's probably an actual leak that's not just air. Could be a worn master cylinder, a busted hose somewhere you can't see well, or a wheel cylinder that's leaking. In that case it's better to take it to a mechanic because if your brakes fail on the road it's a mess. But before you spend money on professional diagnosis, try the bleed, a lot of times that's all it is.