The risk you run with any NOFX album is that it's almost guaranteed to be 50% garbage, but I have fond memories of Punk in Drublic (disclaimer: I haven't listened to it in over 1/2 a decade, at least). I know you already listened to at least a few song off So Long...
Other bands, you've got the well known guys like Anti-Flag (they've always been catchy as fuck, but American Fall has some next-level ear worms), Bad Religion (Generator through Stranger Than Fiction are probably their strongest albums), Pennywise I know you've listened to, Strung Out has some good stuff (An American Paradox was a favourite of younger Geoff, and revisiting it now, it's still a fun album).
Good Riddance leans a little more towards the hardcore end, but they're still one of my favourite bands. I'd recommend checking out "A Comprehensive Guide to Modern Rebellion" and "Ballads from the Revolution"
The Lawrence Arms (and associated acts Slapstick!, The Falcon [1/2 Lawrence Arms, 1/2 Alkaline Trio members], and Sundowner) are pretty great. Apathy and Exhaustion probably has their best "hardcore" songs, whereas Greatest Story Ever Told is more balladic, but a stronger album overall.
Technically Skacore, but I'd be remiss to not recommend The Flatliners among this cadre. The Great Awake is a classic album. Their more recent work is good, too, but it's honestly more of the same. They have a sound, it's good, but maybe played out by now.
Oh! One last suggestion, the Canadian group Propaghandi is absolutely worth checking out. Their album Supporting Caste is perhaps the best thing to come out of Manitoba.
That's probably a good start. Hopefully you finds something new to you and enjoyable.
I'd start with the Lawrence Arms and the aforementioned Bad Religion, then. If I remember them correctly, those albums do a good job of mixing up the tempo fairly well. They tend not to have the silliness of the throwaway NOFX songs, but they do a good job of breaking the monotony of running ahead at full speed every song.
Good Riddance can mix things up a bit, too, (West End Memorial from Comprehensive Guide is practically a drone fugue, and very unlike anything else I've heard from a hardcore group) but as I mentioned before, they tend to be more on the hardcore side of the scale, particularly in their vocals.
Even if you enjoy the Flatliners sound for a bit, I doubt you'd enjoy an entire album. Even their slower songs are still higher energy than 80% of NOFX output.