As a fan of deliberate practice, it helps to first understand where you have room for improvement as a senior engineer. Your feedback reviews at past jobs can be a great place to start.

If you have non technical areas of improvement, you can check out resources like those below to absorb the info and then ofc implement those learnings in practice:

1. Books (eg. Effective engineer, radical candor, Manager's path)

2. Conferences (eg. ELC, LeadDev)

3. Podcasts (most conferences above have podcasts)

4. Coach / mentor

5. Blogs (staffeng, pragmaticengineer, charity.wtf)

On the technical side, it depends on the expertise you'd like to build but here are some general examples:

1. Books (eg those specific to your domain, designing data intensive applications, clean code, language specific books to learn idioms)

2. PR reviews from experienced engineers

3. Blogs (technical blogs of companies you admire)

4. Tech talks (from companies you admire, YouTube has plenty)

5. Courses specific to your domain like algorithms or deep learning)

6. Company conferences (again depending on your domain)