Every District Attorney places his key loyalist as head of the grand jury bureau. That’s the guy who has the power to indict, not indict, and to threaten to indict. That is awesome political AND protective power. The Democrats understand this. Trump never had an Eric Holder.
Ryan and McConnell held all the power. When Trump decided not to remove them, he gave up his presidency. He got nothing with McDaniel, who was perhaps loyalist but not a ruthless strategist. She was merely conventional.
And Trump himself either was not ruthless, or judged that ruthlessness would lead to worse results.
I think he knew he was on a hair trigger in the Senate; that any move too bold our “outside the norms” would lose him the 17 GOP votes that would remove him. There were always a good 25 mush GOPers in the Senate. They would hang with Trump as long as it involved spending money, or “traditional” big government things, but would rebel at any swamp draining.