I’m not sure on the reason the DOJ would indict a member of congress
You're correct on voting objections. For Republicans they see fraud under every lost election (for which they believe they should have won). For Democrats they see disenfranchisement under every lost election (for which they believe they could have won). No crime.
However, the Capitol cop who texted his insurrectionist buddy, the jury convicted him. Likewise I can see DoJ bringing charges against any Coongressperson who gave guided reconnaissance tours for insurrectionists or texted location of Rep Pelosi to insurrectionists in the moment. This is not to limit possibilities such as people like Rep Perry who were actually involved in the plot to overthrow the government.
My feeling is politics will "trump" any real investigation into congressional involvement in the insurrection i.e. politicians are above the law.
