I have the Tamron SP 500mm f/8 mirror lens (55B), and I thoroughly enjoy it. Inexpensive these days, it was always reasonably priced. Sharp and with very good contrast for a mirror. I have the Tamron SP 2X teleconverter, (01F) a good performer, which makes it a 1000mm f/16. I bought it mainly for long shots of distant objects, most at infinity, and birds and aircraft against the sky- bokeh issues are nonexistent for those. The whole setup with cases, filters, everything, cost me about $120.
As polyglot says, avoiding specular highlights, or any distinct highlights, will minimize the donut bokeh. Having the background in shadow, or shooting against a very uniform background will minimize it, too. I have found that a very busy background, like dense grasses, will also work, because the doubling of lines is not so noticeable, and if the background is really busy, the doubling actually has the effect of blending, reducing the effect.
One thing I like about the Tamron is the very close focusing ability. It goes to 1:3, and 1:1.5 with the 2X converter. I can get nice shots close up in which the depth of field is so shallow that the background just blurs into a smooth mass. I have been able to take flower shots in which objects which were out of focus did not display objectionable bokeh.
A mirror can be a lot of fun. Avoiding the donuts means choosing shots carefully, and simply avoiding certain types of shots, but to me that's part of the fun. There is no way I would be carrying a regular 500mm, so the trade-offs mean getting shots with a mirror and dealing with its characteristics, or not getting the shots at all.