Tektro Draco hydraulic disc brake not gripping rotor well enough to stop
As the comment by @Chris H, pointed out, you may have some air in your system. This is easely solvable by bleeding the brakes. Dependening on the design, you may need a big syringe, small hose(ideally translucent) and a small discardable bottle, or a propietary bleeding kit. Hydraulic system needs to be bleeded at least once a years, IMO.

WARNING: Never mix mineral oil with DOT, those are incompatible. You must choose properly your fluid Dot or Mineral Oil. Mixing DOT5 with DOT6, is fine, not recommended but fine.

Check the fittings on the handle and the caliper, tight them if needed. Some air may be getting into the system due to loose fittings. Also check the bleeding valves too.

Lastly, check the hydraulic lines while you pump the lever. If they seem to stretch, wide, or inflate a small ballon, etc. You will need to replace them.