Well
@Grison, you're a lot more patient that I am. You got new bikes and the first thing they say is you should'a bought something else???
These are really great bikes - we have two of them and beyond happy with them. There's nothing to apologize for. You can squeeze over 50 miles out of them, so the range-weenies are full of it. LOL
The Satori seat post works well enough for the price. I have a Harmony, and it's not going to win any awards, but does soak up some jolts at the rear wheel. Mine only moves about a half to three-quarters of an inch.
The stock 'cross-cut gravel' tires are really thin. You can probly get some miles out of them, but there isn't much flat-protection there, and they ride kind of hard. They're really cheap tires designed to deliver the bike and provide crisp riding performance. But a 50mm wide tire just works better on them, Schwalbe has a few options, Continental, something with some flat-protection built in is worthwhile. I have no problems with tires and tubes, can change one out in minutes, so I just run tubes, carry a patch kit and extra tube. I loathe dealing with the slime sealant stuff. Have not had a flat out on the trail in a bazillion years.
Schwalbe has a e-bike road tire that would work really well, and has a very thick protection liner in the carcass. The Marathon E-+ is really nice and will provide a better ride and years of life, along with good flat protection:
I've run Schwalbe Big Apples on mine but they're kind of bulky at 2.35 wide. Just ordered Continental Contact Travel in 50-622, kevlar lining. And just put Schwalbe Energizer Plus ebike tires on the wife's and they're really nice. Greenguard, 50mm, nice heavy duty tires that ride just outstanding. I buy a lot of tires from BikeTiresDirect.com, but they're sold out of these right now but have a few other options. Tires are relatively cheap compared to what they do for you on a bike.
As far as your other items, ToPeak offers some really nice bags and racks, pic of wife's below. The seat-post mounted version are really flexible, offer good fit and quality. A little spendy.
Fenders we don't bother with so no comment. A standard bottle cage works fine. For high-miles or hot weather a CamelBak is hard to beat, and way convenient.
For a lock there are some good options on amazon without breaking the bank. If you want the best they're stupid expensive, like the ABUS and Kryptonite. I'm more than happy with the $35 OnGuard U-lock with cable. Versatile, the cable is a little short, but you can lock the two bikes together or to a rack. And it comes with a mount to attach to one of your bikes. There are a couple of bar-thickness options...the more you pay the heavier-duty they are.
For your Explore, Giant offers some really nice LED lights with li-ion rechargable batts. I have a head and taillight I shift from bike to bike, they work fine, are inexpensive. I also picked up a couple of the cheapo planet bike rear lights and they've been great, 2AAA's in them. We don't really ride at night, so don't worry much, but if you get caught out in the shorter days they're nice to have.
I carry a tool kit I've put together over the years that includes appropriate allen wrenches, a plier, a few applicable wrenches. Nothing too fancy or elaborate. A pump, a patch kit, and a spare tube.
The cool thing about this MTX bag are the built-in saddle bags that flip out. Kind of $$, but cool nonetheless: