There's not that much of a curve. I haven't been in any job where I wasn't fully functioning after at least a year. Plus, there's like 500 of them. Plenty of people spread out the work. Congressional members get plenty of practice legislating at the state level that any new batch of members should be able to come in and do a reasonably good job.
1. Both the senate and the house have about 20 standing committees plus a bunch of temporary committees covering issues from agriculture to homeland security. Most everything that goes to an overall vote has to make it past one or more committees first. Representatives aren’t required to serve on a committee, but are usually allowed to serve on up to two committees and four subcommittees. And even if somebody is on a couple of committees, it’s likely that their constituents would like you to present their issues to the committees the representative is not a member of, and I’d like to think the representative will at least learn enough to have a cogent conversation on those issues. And they’ll need more than a couple of slogans, they’ll need to be able to answer or fend off any pointed questions from the opposition. Being able to do that on all the issues your constituents care about = learning curve.
1a. With all those committees come lots of rules and procedures, and at that level, there are people who weaponize those. They can also argue the other way, saying a rule exists when it doesn’t. There are parliamentarians to help, but they’re more like referees than play callers. Picking up the nuances = learning curve.
2. True there are 435 representatives and 100 senators, but some of them are diametrically opposed to each other, and wouldn’t trust them as far as they could throw them. Plus there are varying levels of competence. It’s not like they have to pass a test, they only get more than 50% of the vote. If the 80-20 rule holds (80% of the work is done by 20% of the people), the vast majority of the work is going to be done by about a hundred people, which will probably break down to about 50 per side. And now you’re going to add the constraint they can only serve a handful of terms? I think you’re asking for trouble.
3. There is no requirement that congressional members serve in any public capacity prior to campaigning for a position. Even those that have served in a public capacity may not have served in the legislature (for instance law enforcement, prosecuting district attorneys, or mayors).
I’d like to think politicians wouldn’t become so entrenched if there wasn’t as much gerrymandering (Republicans have been in the news recently, but the Democrats have done it too). That there’d be more turnover (at least in the house) if districts were more balanced, so that limits wouldn’t be an issue. But Pelosi represents San Francisco, and I suspect any district you draw up there is going to be blue. And you have other areas of the country that, no matter how you draw the district, it’ll probably end up red.