The sandbox campaign is my favorite. However, like you, I can find it to be frustrating at times when players don't engage. At the risk of all of this falling into the category of "Things you already do", here's my two cents:
1. Coinciding with your "PCs aren't special (necessarily)", let their indecision have consequences. You've already stated you have timelines/deadlines etc. Rather than focus on the personal consequences of letting indecision rule, have it impact the setting. This can vary from the minor consequence where a NPC ally is incovenienced to major consequences such as inaction resulting in war, plague, etc.
2. Allegiances/Affiliations - DM-suggestions and borderline railroading seem less like spoon-fed "go here/do this" if conveyed by a superior of their order, commanded by a noble, etc. It doesn't even have to be a "do it b/c they said so". If the PCs revert to their indecisive ways, the assignment is given to another, the PC(s) loses status/favor, has a previously routine request denied them, etc. NPCs can have long memories also.
3. Kickers. The adventure seeks the PCs out, forcing action. This one can be overplayed as much as any other, but it's damn hard to have a debate when people are burning down your house, killing your dog, or trying to kill you. Variation on the theme is important here, of course and it doesn't always have to be life or death. Many of the greatest instances of NPC hatred came from encounters I considered to be throwaways. And for me, nothing reinforces the beauty of the sandbox more than the unexpected transition of a minor NPC into a major one. I've had entire adventure arcs come out of the players' interest in a "minor" NPC once I had to decide what that minor NPC was trying to achieve.
4. NPC timelines. Whether it's an ally, rival, or villain give key NPCs goals and at least a broad plan for how, and especially when, they plan to obtain their objective. If indecision means the enemies are better prepared, greater in number, or higher in level, thems the breaks.
5. In Media Res launchpoints. If none of the above are working or you fear you've used them too frequently, this one can be jarring if your players have never encountered it before. Throw them into a situation/encounter and only after it plays out fill in the backstory of how they arrived at that situation. Maybe their idle lifestyle burned through their loot and they needed work. Maybe after months have passed with little activity they are caught unawares by an old foe. Or a flood wiped out the village or town or fire ravaged the city and now they're homeless.
In my experience, usually 1-4 are enough. When they're not #5 almost always does the trick. It seems to work b/c the amount of unknown information jars them out of their indecision or they're annoyed that the rug was pulled out from under them (even if just a little). Either scenario is a good intro to force a candid conversation. Sandboxes only work if the PCs interact with it. I've found the key is to make the amount of time between "when last we saw our heroes" and "right now" a long enough span that it shocks them a little. My players sometimes get mired in the hour-by-hour, day-by-day play that it's hard to advance the timeline. It drives them nuts when they ask "during the break could I have been planning x". Sadly, no it's a missed opportunity - but you can start doing that now.
Also, to steal an idea from my favorite RPG, Conan, in the 2nd edition they put in a great idea for gaining fate points (or action points, extra XP, goodie of choice). Have the players write down 3 foreshadowed events such as "my character is robbed", "caught with another man's wife", and "duel my hated foe". This gives you insight into what they want from the game. If the situation arises and they take the bait, they earn the cookie. If they don't take the bait, no cookie. Aside from letting them have creative input over the direction of the campaign (in a limited way), it also gives you tangible examples of how indecision resulted in missed opportunities.
Anyway, sorry for the long post. Hope at least some of the ideas are helpful.
And, if you're already doing these things, then I guess great minds think alike!