I am currently owning the a7 III, the Tamron 28-75 2.8 and the Sony 85 1.8 which is at the center of your question.

Camera

The decision between the a7 and the a7r model is he first you should make: Both cams share a lot of traits, however there is more difference than just resolution.

Advantages of the a7 III

• Slightly better performance in low light due to bigger pixels
• Better coverage of AF points and generally slightly more modern AF system
• Better suited to video as it does not use pixel binning and uses the full frame

Advantages of the a7r III

• Better resolution (a plus for product photos) and pixelshift as option
• Better viewfinder and rear monitor resolution (that one is REALLY noticeable, you can barely judge sharpness on the a7 III)
• You can use the APS-C mode and still have enough resolution to use the shot.

So you have traits that speak for each model. The a7r seems to be the better choice if you tend to the still photography, while the a7 is clearly the better choice for video.

Lenses

The Sony 24-105mm f/4 is a glorious lens, but as you already found out, the limitation is in the aperture. If you plan getting the 85mm and the 28-75mm it does not add very much to the mix.

Sony 85mm

This lens is just awesome - especially for the price. It is razor sharp, light, ok bokeh, super fast AF, good build quality. You cannot go wrong with this one.

Tamron 28-75mm

Another very good lens. The cheap price is ridiculous for what you get: Very good sharpness through all the range, fast AF, ok for video, light, good build quality. Drawbacks are the limit to 28 instead of 24mm on the short end and the mediocre bokeh, when you shoot wide open. Still the best lens in that range for the price. And the nearest focus distance of that lens is so small, that you can literally bump into your subject. Which is really really cool.

Note: There is a new Sigma coming out, which also is supposed to be very good. But as with all Sigmas - it will be heavy.

Personal thought

I would propose to buy the Tamron and the Sony 85mm. If you can only afford one lens, pick the Tamron first. 75mm at 2.8 is a decent focal length for portraits as well. The 85mm will supply better bokeh and better sharpness, but is less flexible.

Then as a secondary step, I would evaluate what you have missed most during your first shots and then either invest into a wider lens, or a 70-200 2.8 (Tamron is planning a 75-180mm 2.8 in Spring 2020, it remains to be seen if that one is any good) to expand your options.