Something I don't get about these sprawling multi-year campaigns when using D&D (any edition).
How are the PC's not like 100th level?
If you play, say, 40 sessions/year (weekly sessions planned with 12 weeks off per year, so, like 3/month) of 4 hour sessions, that's 160 hours of play per year. Times 10 years, that's 1600 play hours. If the PC's are 16th level, that means that the characters are leveling up once ever 100 (?!?!) hours of game play? Like one level up ever six months of play? Even if you're concurrently running 3 characters, that's still only leveling up every 33 hours of play.
My players would strangle me if I tried to slow things down to that much of a crawl. Eight or ten sessions to get from level 1 to level 2? And that's the fastest pace? Yikes. How do people do it?
The trick is to make it abundantly clear to the players right up front that levelling up is just an occasional and infrequent side effect of ongoing play rather than the driving reason behind it, and thus not to expect it to happen very often.
Then, there's numerous little tricks you can use to slow down the progression:
--- tweak the xp tables such that advancement isn't nearly as fast as any WotC edition, but also not as slow as RAW 1e or 2e
--- don't give xp for treasure
*** either bring new or replacement characters in about a level below the party average, or at a set floor that very slowly rises as the party's overall level increases (e.g. once the party or campaign average reaches 4th, new characters come in at the start of 3rd)
--- encourage players to build stables of characters and cycle them in and out of play, rather than playing the same one or two all the time
--- encourage players to have their characters get more involved in non-adventuring activities at higher level - politics at the civic or regional or national level, stronghold building and-or maintenance, building a home and starting a family, even just retiring to live the high life - which take time away from adventuring and thus slows their advancement
--- take a page from 1e and have level drain be a thing; also have it that even if restored, you don't get back everything you lost
--- after things get to mid-high level, and if the players are keen (IME they often are), shake it up once in a while by starting a new party at 1st level within the same setting/campaign and running it for an adventure or two. After this, the players will very likely have more characters to add to their stables (and these characters can always be background-adventured up to match the main groups' level if needed), meanwhile the overall campaign progression has stopped dead for however long it took to play out the low level group
*** run different parties concurrently in game time. Party A could be dealing with an evil temple across the sea while Party B (same players, different characters) is seeing to the old wizard's tower in the hills near home, with both adventures happening in the spring of year 1084 and maybe even connected to the same overarching plot line. Run A first, then once that adventure is done put theat party on hold and run B. Once that's done, and if the players want it, let the parties meet, merge, swap characters, etc. and see what parties emerge from that.
The most important and effective tricks are, I think, the two marked '***' above. At low-mid levels, replacement characters coming in at lower level than the average is a very effective anchor, while running concurrent parties is very effective at higher level once they've each got enough characters (and once the campaign has developed enough plots and sub-plots) to make it work.
My current campaign, using modified 1e rules, is 1144 sessions in, over 18 years as of this month; this includes some time running two nights a week (different but connected PCs and players). No single character has been in more than 275 of those sessions, and no single player has been in more than 700. The highest level is 11th, with one of those now able to see 12th approaching in the distance.
The game I play in is, if anything, a bit bigger: very similar rule-set, about 1165 sessions in, over 19 years (again as of this month). There, while there's a few individual players who have been in almost every session the highest single character session count is about 650 and the highest level is 15th.