One of the “must have” accessories for your TS-850 is certainly the DSP-100 Digital Processor, I was lucky to find one second hand years later it was discontinued.
But, what really makes a big difference are the set of optional crystal IF filters and the more recently added Roofing Filter.
Some years ago I heard about the filter manufacturer INRAD International Radio and I purchased some IF crystal filters for my TS-850. The reception perfomance improved dramaticaly. Those were filters easy to install, just plug ‘n play units.
Later, INRAD launched the roofing filter kit for TS-850, among other veteran equiment, that did not have the possibility to have roofing filters from the original manufacturer.