As to the game difficulty, more of the stats seem to be in the gear than in the levels imo. I left my main team fairly high-level (on the cusp of 60) to go train my other party. I also actually did the side content training my second party (to make up for not exploring the map while getting places). So I got a lot of great equipment from doing those sidequests and just looting even more final dungeons. Coming back to my main party with all that gear...Olberic jumped from doing <20K buffed & debuffed Brand's Thunder to doing 35-40k, for example, which is, to put it in context, almost half a chapter 4 boss's HP pool.
Now, a lot of that gear I could've had for a long time--I could've been stealing and doing side content the whole game; Olberic's chapter 4 (my second) unlocked that super sword that really changed things--but I didn't. I was just scrounging and the game had a nice difficulty. I had some trouble with two of my first four bosses, the first and second (although I ended up going back to it last and the gear/levels I got in that process solved the issue for me).
For my second four, I had a ridiculous surplus of great gear that kept accruing. I didn't use all of it--I kept my best armor on my mains--but I did use an important amount of it--I used my best weapons and second best armor. I ran them through somewhat underleveled against the guidlines--more so than my first party, which had Olberic/Therion lower than the guidline, Ophilia just about at the guideline and Primrose over it by a good amount (until the chapter 4's accrued and they all ended up in the upper 40s, lower 50s by the end of that)--but as long as I wasn't taking one-shots and stuff, my offensive power could cut through the content extremely well and the last four chapter four bosses were mostly non-events.
As were the extra job bosses. I had struggled with them (well, one of them) before going to my second four, and they had just seemed above me. I was taking ridiculous damage. I didn't try too many times and instead did the level 55 side-dungeon in Everhold, wondering what that held. I came out with my first party in the upper 50s and then decided to start my second party, thinking to ping-pong between it and trying the extra job bosses. I didn't really do that in the end, and pretty much just did the second party and sidequests.
So when I got back to them I had an over-leveled party and an over-geared party that just trashed them. It was kind of unfortunate and I feel I missed that experience.
Honestly, the most fun I had with the game was probably when I broke off from leveling everyone, made my first party, and climbed through the chapter 3's sequentially, starting with Olberic, then Therion, then Ophilia, and then Primrose. Chapter 4 was fun too. This is when I truly fell in love with the game--I liked it before but I was anxious about how to consume the content best and unsure of my play-style.
The second most fun I had was actually seeing the world with my second party and, again, going through their stories in a focused fashion, even if the character growth versus content pacing wasn't as thrilling.
I kind of wish I'd blended my two playthroughs better, namely, I wish I'd done more sidecontent at first, explored the villages for more gear at first, but still leave some for the second quest. The reason I didn't is I was honestly made anxious by having Primrose stuck in my party, namely, she was always going to be overleveled if she did all the side-content and that made me want to do only what was immediately in front of me and come back and do the rest later. This is what propelled me to choose one party of four after being noncommittal for quite a while there. I like that decision to focus. It made the game more manageable and also gave me the experience of playing the game twice (going over things again) within one playthrough, which is cool. I probably wouldn't have made that decision if I could've swapped Primrose out, or at least I'd have been slower to do so. Basically, I'm of two minds on this design decision of theirs.
