This is not a good solution - expect bottlenecks
This is to be expected when using a removable device.
Perhaps - but it is the only way to remedy your issues …
I have no idea if disabling write cache will have any effect - you can try though.
An entirely different approach is to use ventoy - but as you say - you have no experience with Linux - and in that sentiment - there is no turnkey solutions - only hard work.
Using ventoy alllows for - besides the ISO storage partition - one or more data partition (use GPT if you want more than one) on the USB device.
Either create your own ISO or use the default full ISO to boot - always save data to the designated storage partition.
The only drawback - unless you create your own ISO - is that you cannot install new applications - you cannot do that with your own either - what you can do is to customize the ISO with your preferred applications.
Perhaps you can gain something by using grub’s copy to ram - it would depend on a lot of factors - and would likely require some experimentation to achieve something useful.