Several questions regarding dual boot system, Fast Boot and Secure Boot, etc

I have a dual boot system with Ubuntu 26.04 LTS and Windows 11 installed on my Galaxy Book 2 Pro 5G. Fast Boot and Secure Boot were disabled during the installation process. When and under what circumstances can I enable them, or what can’t I enable them? (I only have one SSD with one NTFS shared partition, that’s all.)

^{My SSD}

My second question is, if I remove Ubuntu from the dual boot system and reinstall it, do I need to modify GRUB or any other settings?

My third question is, the ESP partition is currently about 200MB. What are the benefits of increasing it to 500MB or more for a dual boot system, such as speeding up Ubuntu’s boot time?