I might try for ju-jitsu - a pair of related moves that aren't directly very aggressive but have synergy together.
I'd look into trading Paddack plus some minimal prospect capital for a good fielder at SS who has an adequate but subpar bat and a low salary, i.e. a backup who's better than waiver wire fodder.
At the same time I'd look at trading Castro to someone else for a capable but probably not stellar relief pitcher at a low salary plus some minimal prospect capital coming back to us.
The combined aim would be to have a slightly different mix of skills at the utility infield position (Willi is no great shakes at SS while his bat is overrated anyplace else), and in effect redeploy Paddack into some other relief arm at lower cost, since that could turn out to be his best use anyway.
It gets out from under the two not-terribly significant salaries that seem to be such an issue, at perhaps a slight downgrade at the two positions, but we have minor-league candidates for both roles who might be better and would surpass the two incumbents soon anyway. For decent return at the major league level, we might have to take a net loss on the prospects going out versus coming back, but nothing remotely like a top-ten prospect leaving unless something similar comes back.
I don't think you could trade Paddack for pitching, nor Castro for infield help, directly. But the desire to trade both of them creates opportunity, without the need to orchestrate an actual 3-team trade yet with the same end result.