How do I become a marketable frontend engineer?

The nice thing about front-end is that it's inherently demonstrable. Given your backend experience, I'd suggest you apply for generic (full-stack) engineering roles, unless the company you're applying to is forcing you to pick a track. Once you get senior at a large company, it should be easy to then specialize in what you want.

If your goal is senior eng at a larger company, I'd focus on showing your projects/influence at high levels.

• Do you have a blog post, publication, or public launch of your work at Mastercard?
• Do you have a legit side project that looks good and also has a non-trivial number of users (1K+)?

In general, the first option is better if you're going for a senior-level role, since that shows your ability to create impact in a large company. The side project option is harder to really stand out since there are many good-looking projects out there. You need users to make it truly impressive (learn more about this here).

I don't have a strong opinion about React vs Angular, pick what you'll be most productive with!

I want to join a proper tech company next year (outside fin-tech) as a senior front-end engineer.

I actually have a much more tactical piece of advice here: Get to a senior level at Mastercard first. Here's why:

• It is difficult (and risky) to negotiate an up-level, especially from mid-level to senior.
• There's generally a large mentality shift required to make this level up which I talk about in-depth here. You don't want get an up-level through a job search and become a low-performing senior engineer (I'm interpreting "proper" tech company as something that's on the level of FAANG).
• The current economic climate also leans much more in the favor of the hiring companies vs. the interview candidates, so it will be very difficult to get a senior offer if you're still currently mid-level.
• I talk more about all this in our video about how promotion is the foundation for your career.

All that being said, I'm really glad you like front-end (I do too!), and I think side projects are a super powerful career level for those who love putting pixels on screen. I've gotten many interviews from "proper" tech companies like Instacart, Block, and Uber through my side projects, and I recommend this masterclass on how I did that (this one's about the execution, and Rahul linked the one about ideation).

I agree with Rahul in that you can apply to full-stack roles, but I recommend doing front-end only projects to deepen your focus. Adding a back-end to a side project greatly increases its complexity, and you like front-end a lot anyways. 90% of my side projects were front-end only (just ran locally on Android). If you're going to call an API, call someone's else - Don't build it yourself. Using the project to learn React seems like a good idea too.

I am good at debugging (cause that's what most of my work is recently) but I don't know how that can transfer to other jobs.

Debugging is always a useful skill (spending most of your time debugging is a Staff Engineer archetype), but it's important at your career stage to get feature work as well. If you aren't spending 35%+ of your time on that, I would talk to your manager about procuring more.