Interior photography - Upgrade from Sony α900

Hello! I'm new and would like to welcome everyone.

I really need a piece of advice and I'd appreciate it if someone could steer me in the right direction.

Although I'm not a professional photographer, I enjoy taking pictures because it helps me grow my business. I run a bespoke furniture workshop. We are producing all kinds of built-in furniture like kitchens or wardrobes. The majority of the projects we complete are luxurious, so I'd like to have the same quality level of photos to show my other clients.

Right now I am using the Sony A900 with a second hand Sigma 12-24 mm f/4.5-5.6 EX DG
I've come a long way to finally achieve a reasonable quality of my photos, but there are a lot of thing that should be improved.

Here is a list of the existing issues I am experiencing:
1. I think the lens I am using is not so sharp. I don't think there is a point in investing in a dead A mount system. In my country there are really not many offers for those lenses.
2. This camera doesn't have a live view. I am usually shooting from tripod on my chest level. It's terrible for my back to get to the viewfinder all the time. Not having a live view is maybe nothing bad for the pro photographers, but how can I know where the sharpness is, etc. It's so time costuming to go to gallery and check each photo.
3. I can't spend as much time as I would need at customers house. I don't want to sit on somebody's head the whole day.
The problem is that this camera is a little outdated and it's not so easy to shoot remotely. I have to use many 3rd party equipment. The Sony remote doesn't work good. For the Godox flash I have to use Canon shoe adapter + sync cord. Many cords and Chinese equipment that sometimes stops working during the session.

Conclusion: I know many people say that Sony A900 is still a great camera... maybe it is for pro's, but for me as a part time photographer is just a hard camera. Or perhaps is just for the type of photography I am doing.

I think that I don't want to spend a lot of money for the new gear since, I don't take pictures for a living. However, I really enjoy technology and my photos will generate some kind of an income.

In short words - what would you recommend for interior photography (kitchens, small and large bathrooms, rooms with huge windows and lots of natural light, rooms without nature light, etc.) Additionally I would like to start taking a "studio-like" photos of my smaller furniture projects. 
Regarding the outcome for the photos - I am using them for social media, my website. However I would love to use them for a real printed brochure to promote my company. Although for that one I would have to improve my skills.
I also have a Sony A6300 as a "hobby" camera, it's great but maybe full format would be a better choice in therms of quality. I think that I would like to stay with Sony. I know the system so that a plus.
I am looking for a camera and lenses recommendation. Personally I was thinking about Sony A7 III, but I am not so oriented in lenses.

Perhaps I am just terrible photographer and blaming all on a camera. You tell me

Here is an example of my skills.