Ok an update on this, I continued the diagnosis elsewhere, but it turned into a bigger problem.
First I now can always get LTE on both phones (or 5G icon but not 5G itself on the newer phone aka fake 5G).
Second the older phone now has the slower speeds as well in late afternoon and evenings.
Now on the more interesting bits, I would think people are interested as is lots of mobile ISP discussion here.
In busy hours the EE mobile connection does two things.
1 - The download speeds drop of considerably but are still at reasonable enough levels I would expect to browse on the phone. (circa 5-20mbit). However they have a clear pattern of traffic shaping, where they initially spike, then flat line at a low speed.
2 - The upstream speed caps out below 1mbit, when its normally 50-70mbit.
So I did a couple of things.
I got a 1P mobile sim to compare the performance, the EE sim is on the essentials package which is supposed to be capped to 100mbit, but I cannot find any mention of additional limiting during peak hours in the package spec.
When testing the 1P sim, not only does it not slow down in the evenings, but also is consistently faster in the day as well (just to a much less extremity).
As an example
5.6 down, 0.74 up on EE, 141 down 74.2 up on 1P, carried out same time on same phone (dual sim phone).
These are repeated tests, repeated results. Off peak the EE gets close'ish on the upstream but fairly consistently seems to be about 50-60% of the 1P throughput on downstream.
I discovered my local cell mast only has band 3, and compared to all other cells in 5 mile radius is by far the smallest.
When I disabled band 3 on the phone, I was connected to one of the bigger cells further away, and outdoor performance during evenings sky rocketed on the EE sim, but I have to be outdoors due to weak signal.
The local cell is across my road, and I get max signal indoors, with no boost of speed outdoors at all.
So the current most likely outcome looks like there is congestion on my local EE cell, and they have decided to mitigate it by traffic management. I suspect the top EE packages, will perform the same as 1P. I only moved to essentials about 6-7 months ago.