The DS situation is a little weird to be honest. We basically got him for nothing because of a lucky situation this offseason. He has value to a team that wants to make a run. As of right now, the Celtics don't look like a team capable of making a run, however we have two All Star players that need to feel like they've got the management behind them. Trading DS for a second round pick, despite being excellent asset management, might not make those guys very happy, and could be the beginning of a bigger issue down the line. Additionally they might completely understand the move (I can't read their minds, but it's something to account for)
I think if moving DS before the trade deadline allows you to make moves that could lead to a bigger move that actually moves the needle this offseason or next year, then you do it. If all you're getting back is a second round pick though, with very little ability to do more (like was stated, using the second to also move Hernangomez and then using that added flexibility to make an additional bigger move) I think you keep him.
I agree that getting essentially a "free" second round pick for Dennis is good asset management, as he basically fell into our laps, but if that second round pick can't be used to make bigger impact moves, then I think goodwill with Tatum and Brown and saying "we live or die with how far you guys take us" has more value long term.
tl;dr - if trading Dennis is the domino to make bigger moves going forward, trade him. If not, keep him to say to the Jays that we are behind them.