I see the risk as one that increases over time. Trump demands loyalty to himself personally, not the country or Constitution. People like Mattis, McMaster, Kelly, etc can only take it so long, and they're getting replaced with the kind of "yes men" Trump's ego requires.Theoretically, yes its a huge risk. With Trump, we now have 4 years worth of track record and it doesn't seem like he himself really presents that risk. JMO, but I think the reaction to him has created a much bigger risk to stability and national security than anything he has done.
The single biggest check on Trump right now is re-election. There's reporting that he wants to fire Wray, Haspel, and Esper if re-elected. That's nuts. Those are all his appointees, but he wants people who will publicly defend him and harass his political enemies in those positions. Even Barr hasn't done enough to get Trump's approval.
A second Trump term eliminates the single biggest guardrail on his presidency - re-election. It also validates his efforts to politicize the DOJ/FBI etc as being affirmed by the public. So right now, there are a bunch of people around Trump advising him on the electoral implications of various things. Whatever counter-weight that provides to his actions is gone if he's re-elected.
So whatever risks are present in the first 4 years are amplified by some factor over the following 4 years. I'm not saying Trump is going to try an overt power grab, but IF that was the end game, your strategy for years 4-8 would look much different than years 1-4.