Welcome. What about your current setup is not meeting your needs? If it is that you are having to use high ISOs, I'm afraid new equipment will do little to help with that. In general, a full frame sensor will give you better low light performance than a crop frame, but the difference isn't huge. The only other equipment that will help is a faster lens. You are already using a fast lens, but you could move up to a 50mm 1.4 with you Pentax. That would cut your ISOs by 1/2 stop. Unfortunately, there aren't a lot of Pentax mount lenses available for Pentax digital, as opposed to other more popular brands, such as Nikon, Canon etc.
What is your budget? A lot depends on that.
Unfortunately these days, starting over with a different system can get expensive, especially if you need higher end gear. Without knowing your budget, I would recommend a Nikon Z6iii and a 50mm F1.2 lens. That would run you around $4,000 new here in the US. If that is beyond your budget, there are other options that might suit your needs for less.
Do you own, and use other lenses? Having to replace a number of lenses, obviously increases the amount of money needed.
The other option is to use your current equipment and make changes with how you shoot and edit. That could also be a stopgap while saving for the equipment you want.
Are you using noise reduction software? This can make a huge difference in your images. This hummingbird image was shot at ISO 8000, and then somewhat heavily cropped.
I processed it using Topaz. There are other options. If you are already using Lightroom, the latest version has rather good noise reduction, albeit, a bit slow.
The other thing is to use slower shutter speeds, if possible, to bring ISOs down. A lot depends on what exactly you are shooting. Obviously, moving subjects require faster shutter speeds. Static shots, like landscapes, can normally be shot at very low shutters speeds using a tripod, and give you low ISOs even in very low light situations.
You may already know much of this, and apologies if you do. Just trying to hit the obvious things as I don't know your experience level and shooting style. Showing us some of your work and what you are trying to move forward with would be helpful.
The other thing is that this is a Nikon forum, so the advice you get will mostly involve Nikon equipment. Everyone makes good cameras these days, although some excel in some areas over other manufacturers. You have to figure out what system fits your needs and budget.
Good luck, and we look forward to seeing some of your work.
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