First, D76 can be reused several times. It is "same" developer as Ilford ID11 and some other developers (Efke FR4 for example), and it can be considered as "universal" film developer. For example 1 liter working solution of ID11 can be used for developing 10 films, and it is sugested in developer instructions 10% longer developing after every film. For example if you make fresh developer and develop 1 film 10 minutes, for second film you need to develop it 11 minutes, third film 12 minutes etc... But, check D76 instructions for that. Fixers, stop baths and wetting agents are made to be reused.
Only if you make dilutions meant to be used only once, or developer which is manufactured to be used as "one shot" developer, then you can't reuse developer and after developing you must throw away developer and make fresh next time you develop film.
About particles: I only get black particles in my fixer. Never in developer or on developer bottle inside walls. And they built as black "coating" on inside walls of bottle in which fixer is. They don't influence fixing. These are residues of silver remaining after fixing. I just do next: After every processing I return fixer in bottle in a way that I put coffe filter into funel and pour fixer through filter back into bottle.
As for silver particles: See film if there is emulsion falling from film. If that is case, then it is film problem. If that is not case, I think you didn't mix developer properly. Make sure that all powder parts of developer are completely dissolved in water when mixing developer next time. Folow developer mixing instructions. You can even buy new bottle if you want to be sure, they are not expencive. For example I payer certified chemistry laboratorry bottle for 1 liter, brown, glass, 2 EURO (about 2,30USD).
And make sure there are no temperature differences of chemicals. For example make sure that if developer is 20 Celsious degrees, stop and fixer should be 18 to 24 degrees. Don't let developer to be 30 degrees or 10 degrees, fixer 40 or 5 degrees, water -10 degrees and like that...
But, I really can't figure out what silver coloured particle really could be...