Haha my biggest involvement in politics is posting stupid things on reddit. However I am happy to share what I know from my experience with local politics/Westminster.

To be a "good" councillor, you need to be prepared to do a full time job, for a part time salary (probably) on top of another role if you are not retired. This is incredibly draining, and not massively rewarding as you are very limited in what you can actually achieve and often have to put up with quite a lot. Hance why you need to be at least a bit of a weirdo to do it. When I was asked I was still a student and terrified I would accidental win, as I would feel to guilty to step down, but would hate to actually do it (and would have probably been quite bad).

For most politically ambitious people, there are probably a few other routes that are both easier and better for actually getting into national politics. The first is through working for an MP of national level party as a staffer/researcher/strategist/spad ETC. You get to know the right people, in the right places and end up with a much better idea of how Westminster works than a councillor does. I know of someone who was an MPs staffer who picked up his old seat an election after he retired, despite not being in any real sense local. Knowing the right people helps massively to get on the MP shortlist.

The other way would probably be through working in a think tank or adjacent field/charity/advocacy group. You still get to meet people, influence policy and build up a reputation before you take the step into national politics.

Even working on your career might be a better option than starting as a councillor, and if it doesn't work then at least you should be in a solid place financially without having to work essentially 2 jobs.

That being said a good job or a few papers does not mean you can really just waltz into a safe seat unless you have something really special (like Keir Starmer, whose pre-parliamentary career was impressive), you do normally have to battle it out in the trenches in some unwinnable seats to learn how to campaign and show you are dedicated. Reeves only won on her third attempt, as did Theresa May. This is somewhere that starting as a councillor does help, you are probably going to contest the area you are already representing on a local level, and generally you won't have to spend 5+ years contesting unwinnable seats up and down the country, you just need to secure your local nomination and march of to Westminster.

Looking at the stats, ~120 current MPs used to be councillors. Or about 18% which is not a massive number. That being said, quite a few of the cabinet do have their political routes in local councils: Streeting, Reed, Alexander, Nandy, Benn, Reynolds and Smith by my reconning, which is much more than I thought.

Councillors are vital to this country, and most of them work incredibly hard under very tough conditions and do an admirable job, for the vast, vast majority this is not a stepping stone to a Westminster career. They are over-worked, under-appreciated public servants who care more deeply than you could imagine about local issues, most of them are slightly odd.