I have an old Mac Pro 5.1 and it is a little slow I am sure compared to new machines, so a render of 3000 images can take hours !

I see the internal render allows to render an already created Intermediate JPEG sequence.

My initial test renders I do in 720 or 1080 Low Quality.

Can I then later use this same JPEG Intermediate sequence to render a 4k HQ video or are the JPEGs exported to a higher quality at the better render settings?

The JPEGs exported seem to be the same size, but I just wanted to check.

Yes of course! That's the whole idea behind the lrtimelapse export and render.
If you open the render dialog, just switch the "render preexisting sequence (from lightroom)" and select the preexisting intermediary sequence (prefix LRT_).

So the exported JPEGS are exactly the same regardless of Video quality setting

Thanks
Will save me a lot of time

Exactly.

Great. I have used the existing JPEG sequence in the past, but only at same quality and used Overwrite to change a few frames.
It is great not to have to do the export to JPEG each time