I currently own the A7 III, the Tamron 28-75 2.8 and the Sony 85 1.8 which is the focus of your question.
The decision between the a7 model and the a7r model is that it must first take:
The two cameras share many features, but there is more difference than the simple resolution.
Benefits of the A7 III
- Slightly better performance in low light due to larger pixels
- Better AF point coverage and generally slightly more modern AF system
- Best suited for video as it doesn't use pixel binning and uses full format
** Benefits of the a7r III
– Better resolution (a plus for product photos) and optional pixel shift
– Better resolution of the viewfinder and rear monitor (this is really noticeable, you can hardly judge the sharpness on the A7 III)
– You can use the APS-C mode and always have sufficient resolution to use the shooting.
So you have traits that speak for each model. The A7R seems to be the best choice if you tend to shoot still, while the A7 is clearly the best choice for video.
The Sony 24-105mm f / 4 is a magnificent lens, but as you have already discovered, the limitation lies in the aperture. If you plan on getting the 85mm and the 28-75mm, that doesn't add much to the mix.
This lens is just great – especially for the price. It is razor sharp, light, ok bokeh, super fast AF, good build quality. You can't go wrong with this one.
Another very good goal. The cheap price is ridiculous for what you get: Very good sharpness across the range, fast AF, ok for video, light, good build quality. The downsides are the limit at 28 instead of 24mm on the short end and the poor bokeh, when you shoot wide open. Always the best lens in this range for the price. And the closest focusing distance to this lens is so small that you can literally fall on your subject. Which is really cool.
Note: There is a new Sigma which is also supposed to be very good. But as with all Sigmas – it will be heavy.
I would suggest buying the Tamron and the Sony 85mm. If you can only afford one goal, choose the Tamron first. 75mm at 2.8 is also a decent focal length for portraits. The 85mm will provide better bokeh and better sharpness, but is less flexible.
Then, as a secondary step, I would assess what you missed the most in your first shots, and then invest in a larger lens or a 70-200 2.8 (Tamron plans a 75-180 mm 2.8 in spring 2020) to expand your options.