As Jim, W0EB, accurately stated, there is not much room in processing resources with the single, stock Nano. It takes almost ALL of them for either the v6 stock graphic display or the KD8CEC (correct call sign) functions with a 4 line display without going to an outboard graphic processor (that is what the Nextion screen adds, and why it is more expensive). There is one program change that can be made that does not take any hardware (hw) changes. That one was listed by Dave, WD8AJQ, earlier in this thread.
If you want better graphics, then you will need to invest in a processor, either Nextion screen type or Teensy general-purpose type. The choice should be on what you want to do with it. If you just want to operate then look to Reed N's firmware, as that will improve a number of things like screen update time and CW operation. If you want the features of the KD8CEC and do not want to get into heavy programming, then the only option is to buy the Nextion screen. If you want to have the extra processing power to do things with the processor like audio filtering, CW decoding, linear amp integration, ATU, and other functions that I just cannot think of right now then Jim's Teensy route is the way to go. The first two options are either a download or a hw mod and download. The last option has enough hw capability to allow you to add your own programs to the code. As a side note, Jim is updating the code on a somewhat frequent basis, so go look at his webpage for current software features and capabilities.
I should note one thing about the KD8CEC route, there is an extension to the processing capabilities by adding another Nano linked with I2C communications. Now you are investing in more hardware and hacking the Raduino to get the I2C signal. It adds the capabilities that I mentioned with Jim's Teensy solution. I have not done a cost comparison to have an opinion. There are also up to 7inch screens available.
Above are my thoughts on reading most of the posts in the BITX20 groups.io and having built or modified many uBITX from v4 to v6. I am ALWAYS open to suggestions for improvement, or corrections where I have made a mistake.