Is your goal to build products for other people? Or to do prototypes yourself? That's kind of the key question.
If you're going to do embedded professionally,you'll need to graduate from Arduino sooner rather than later,because you won't fully learn the chip until you're programming in a regular development environment and building the lower-level parts yourself,and have a full toolset. Moreover,you can probably stick with Arduino and just learn from doing a bunch of projects and trying to write more of your own libraries.
Personally,I am not a huge AVR fan. If you decide what you are like to work on next,you should pick a processor family based on the available peripherals and vendor support. I'd tend towards an ARM Cortex-M0,M3,M4 or M7 from Cypress,Ambiq,SiLabs,TI,STM or NXP. But mostly,focus on what you need from the microcontroller beyond the core-- Bluetooth,Wi-Fi,ultra low power,high accuracy ADC,high I/O count,and low cost.
Cypress is my current favorite for their PSoC processors. I like their robust but easy to learn IDE,the reconfigurable analog and digital hardware and the approach of inserting library source code into your project rather than just to link it to a binary library or forcing you to use a particular RTOS.
But I can promise you have not "learned microcontrollers" yet. That takes many years and a lot of experience with multiple microcontrollers,system designs,and development environments. You're kind of just getting started,and there is plenty of fun ahead.
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