Dag Humma, welkom to the forum and happy holidays.
First of all the K5 is a very capable camera - no doubts there. Your situation is a familiar story. We all want natural lighting as much as possible. But a flash certainly helps and of course you can modify the light in various ways. A budget solution is bouncing flash off the wall/ceiling, a diffuser or other accessory is also useful. Lookup Neil van Niekerk who says: "lighting in photography is all about the
" and his other video about the
black foamie thing light. Hope this will get you thinking.
You may also like to invest in some faster lens. A relatively good budget manual focus lens is the M50/1.7 prime which you can pickup on marktplaats.nl for less than EUR 25. I like the FA43 Limited, but its pricey or the FA35.f2 or the FA50/1.7. I like FA lenses (all with AF) but I am biased - there are lots of other AF/zoom options out there too. Personally, I would not jump ship to Digital FF until I have explored all the opportunities APSC offers. BTW a FF setup is not cheap and doesn't guarantee great shots unless you know what you are doing. beste.