Hello and welcome
Any K-mount lens, whether Pentax, Sigma, Tamron or otherwise, with a dedicated aperture ring and "A" setting on it allows you to set the aperture from the camera's control wheel by setting the ring to the "A" position. Then, from within any of the P, Sv, Tv, Av, TAv, M and B modes, you select the aperture of the lens using the control wheel on the camera. For lenses without a dedicated aperture ring (e.g. Pentax DA lenses), they operate as those with a ring set to the "A" position.
I don't know what Pentax and Sigma lenses you've been using. Generally, I would say most modern Pentax lenses are great. Older models vary, but are mostly OK-to-excellent. Sigma lenses are, in my experience, much more variable in performance depending on whether they're primes, zooms, and the era in which they were developed and released. Generally, I prefer Pentax lenses. But there are exceptions, such as the DA*16-50, where the Sigma equivalent - in this case, the 17-50 f/2.8 HSM - is arguably better optically.
The latest Sigma lenses can be very good indeed, but those that are compatible with Pentax K-mount sometimes have inconsistent or unreliable auto-focusing, as Sigma reverse-engineered Pentax AF operation when designing its lenses, rather than paying for a licence to use the official specification