How many games *are* you running btw?I am too new to pbp to have a diffident opinion, but I have found the forum here a great release for my wanting to tromp through dungeons and hit things with a big axe. I hope to one day be able to help address your concerns, maybe when I level up! haha
1: It may be futile to try to do a huge level 1-20 (ish) adventure path. The game that we are about to complete took us from level 4 to 7 (It's 99.9% sure we will be finished in 1-2 weeks tops). It was a somewhat sizable dungeon, but *clearly* not a full-sized campaign... and yet this took us almost 14 months in real life! So I think that doing one module, or maybe a series of 2-3 small adventures, is very reasonable, but a full campaign (like say one of the big ones published in 5e, or a Paizo adventure path) is probably not achievable. Knowing that there is a clear, achievable conclusion in sight can keep people motivated too.
2: The start of a game is important. There can be a *lot* of energy and excitement at first, and it's important for the GM to take advantage of it and go with it. This energy can be spent designing the characters and the setting, but I think it's important that not TOO much effort be spent on this, because the start of the adventure can then feel like a let down! So starting the actual game with a bit of a bang can be good. I known it's good to have an easy "introduction-battle" to let people gel, but it can be boring. On the other hand, that bang shouldn't be something too railroady. "Oh that idilic town that you guys spent 2-3 weeks thinking about? UNSTOPPABLE INVASION, RUN FOR YOUR LIVES!!!!"... not always the best. (I've had it twice and both games didn't complete. coincidence?)
3: On the topic of railroad, too much choices can paralyze the campaign, so a clear path forward help things move along
4: Returning to starting a game - I don't think starting at level 1 is great. I think 3 is the minimum, and if I was to run a game tomorrow I would have characters start at level 4 or 5. Fighting kobolds isn't that fun. You don't want to make the players wait to hit that "D&D mid-level sweet spot". If the game won't be super long (see point 1), why not *start* there? Besides, it makes for characters with more robust and varied backgrounds, and multi-class (if you allow it) is more feasible.
5: It is quite important that the party gels well. So some kind of "party balance" is nice (doesn't have to be perfect - heck in one game we have 2 fighters, a paladin, a ranger and a monk!), but what I'm talking about is more that the characters get along and aren't too disruptive. This mean that the "fun pirate who's actually kinda evil and a jerk" can be a problem, the "attack everything" barbarian could be a problem, the "ultra loner" is another problem... Heck I have to admit that I was a bit guilty of that in one game - Udit is a pessimist and he was a bit of a pain initially - I realized that was an issue and toned it down (sorry Fitz!). I have seen games where an arrogant, not willing to play along party member sapped the energy of a game and it fizzled out.
6: Games with intrigue and politics can be challenging to run. Games where the setting is very different than your "generic late-medieval vaguely European" default can make it more challenging for the players to get engage because there is now a "cultural" barrier - however if you can navigate that, it can be *very* interesting.
7: I'm really not sure what is the ideal number of players. Some have suggested over-recruiting to account for player loss/attrition, but at the same time with more people the odds of one player being a bit busy or slow increase, therefore slowing down the game.
The fact that you have completed 2 games and running so many more indicates that you know what you are doing... any games that didn't work out?Let's see...
I am currently running 8 games (1 to come, and 1 will end in a few months), all are modules or Adventure League games. I find it easy because half the work is done for me, all I need do is apply what the characters do to change things when the time comes. I have a black composition book for everyone with notes as to the mid/high levels of the game and things PCs should "find" to progress the game along.
I have had 2 games start and successfully end, in the One Shot mode. And a few I have needed to pull by the hair to keep going lol. Maybe One Shots are what you are looking for? Short games that have a diffient ending and characters you can come back to.
Oh and I have a suggestion for [MENTION=1]Morrus[/MENTION] too... try to see if there is a possibility for adding a forum functionality by which a forum member can subscribe to a thread and get notifications when someone else replies to the thread. AFAIK this is not how it works at the moment, you get notifications only if you receive a PM, mention or XP. Having notifications on subscribed threads would greatly help a PbP player know when a game has progressed, and reach the thread in a couple of clicks rather than having to find it in the subforums.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.