I played some WFRP 1e with some old and new friends yesterday, and after the game we were discussing the game's virtues and flaws, as well as various classic campaigns, and I realized a thing: WFRP is singularly unsuited for mega-campaigns like The Enemy Within.
I take issue with this, but will give my thoughts on this below. First, to answer the question at the end of your post.
My understanding is that with 4e, C7 have released a number of adventure anthologies and maybe other shorter adventures (in addition to the remake of Enemy Within). Does anyone have experience with these, and would these match the actual system better?
I think that the three collections of Ubersreik Adventures, along with Rough Nights and Hard Days can be made into a satisfying campaign with less rail-roading than The Enemy Within. They also are are available fully prepped for Foundry if that is a thing you care about. There are a number of other converted old adventures and new adventures available as PDFs on DTRPG that I don't have a lot of experience with but can add even more variety. Check out One Shots of the Reikland, Night of Blood (a classic one shot), Its Your Funeral, Emperor's Wrath, and the adventures in The Enemy Within Companion Guides.
The Rat Catcher's Guild's fanzine
The Ratter generally has multiple fan-made adventures per issue. You can download them at the Rat Catcher's Guild Discord server, or at the Jay Hafner's external repository here:
THE RATTER – A WFRP FANZINE
The reason is the advancement system. While there are differences between 1e, 2e, and 4e (let's ignore 3e for the moment), they are all based on the same principle: you start out in a fairly low-class career (rat catcher, beggar, pit fighter, scout, scribe, craftsman's apprentice), acquire XP to advance various stats and skills, and once you've done enough of that you can move on to a new career which opens up new advancement opportunities. But these new careers aren't just words on a character sheet. If you want to move up from being a mere trader to a merchant, you need to actually be a merchant: you need to buy a town house and a warehouse for your business, you need to hire some scribes and either a train of mules and drivers or invest in part of a merchant ship, and you need to invest in 2500 GC worth of trade goods (at least in 1e). If you want to advance from Outlaw to Outlaw Chief, you need to actually recruit a small band of other outlaws.
That's pretty cool as a concept, but it's not really compatible with "We need to save the world"-style campaigns where it can be hard to find the time to recruit a band of outlaws or getting a job as a judicial champion. That's the kind of stuff that needs either plenty of downtime or, better, roleplaying out the acquisition of a new career.
I've been running The Enemy Withing for over a year and it hasn't been an issue. I'm only discussing 4e in this post. I last played 1e in the '80s and never played nor even read 2e or 3e.
Per the core rules, you CAN move to a career in another class, it just costs an extra 100 XP. Also, if you you completed a level in your career, you can take the next level of another career in the same class for the same xp cost as going to the next level in your current career. The rules recommend an in-world justification.
Also, their is an implied acceptance of DM fiat regarding this that is seen throughout the adventures. You'll find example throughout the published adventures where at certain points in adventures you are given the option to change to a career in a different class if there are good in-world/story reasons for it without the XP penalty. For example, a section of an adventure where you are working with an outlaw camp, will give you an opportunity to change to the outlaw career for only 100xp even if outside your class and even if you have not completed your current career. There is another adventure with a one-off offer to allow a PC to take level 4 of the Judicial Champion career for 0 xp.
I think that this is another example of where an intended rule or rule-variant was cut out in the final editing. Andy Law, one of the main rule designers has talked about some of the issue with the final editing that has led to incongruities like this. But it can also be chalked up to the Golden Rule. I wish they would have just spelled it out as a variant in the call-out book in the character-advancement section.
I tend to stick with RAW except where there are very strong in-story reasons to allow changing to careers in a different class or skipping career levels for less or no XP. Also, because I'm running a long multi-year (IRL) campaign, I am on the low end in terms of the amount of XP I award per session. I tend to try to find ways for players to get what they want out of their characters, while still keeping mechanical balance and in-story versimultude.
As for the trappings you are expected to have for your career, these can be as challenging or hand-wavy as the story requires. I tend to be a bit hand-wavy on how characters get their training to use their XP but I like to create scenes and scenarios into the story in terms of how they get their trappings, networks, etc. This is something that, in my experience, works very well with a campaign like The Enemy Within, which leans heavily into the social and political aspects of the game.
I personally like the career advancement rules and downtime rules. I think they give a lot of flexability in terms of character build while putting on restrictions that are mostly based on the in-game setting. The system supports the setting. Yet, it also allows for opportunites where exceptional in-world opportunites can help you shortcut career changes and talent acquisition. My personal WFRP GM philosopy is that WFRP is not overly concerned with game balance. I try to keep a cadence where things stay deadly and difficult longer with a slow-burn approach to granting XP, with the occasional milestone jumps and opportunities for shortcuts.