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@gbj_tester
Take it all apart and replace them with metal. If you haven't squished your Delrin spacers into shims, you probably haven't torqued the BB cup enough, and it will precess and eventually fall out, damaging the threads in the process. Even if the plastic ring survives initial tightening, the material will creep in the long term, causing the threaded connection to loosen.
Plastic cassette spacers are not designed for significant axial load, whereas the BB should have about 20kN compression force between the fixed cup flange and shell. Basically, a BB spacer would need to survive being sandwiched between a bottle jack and one corner of a 7.5 ton truck.