Sadly there is no one good network due to the nature of mobile, congestion comes and goes for the reasons you describe, upgrades, people only being there during certain times of year, moving to other networks, etc
I was consistently getting 120Mbit off Vodafone for months, then it halved (still decent granted). I'm right next to the mast however so its WAY more reliable than Three 5G where I can get anywhere between 500-700Mbit but has periods of packet loss (likely down to poor 4G performance which is uses for uploads) which is just not stable enough to use my VPNs.
Basically you have to be very lucky for mobile to get close to the reliability of fixed-line broadband. Which is I why I have VDSL as my primary connection with both Vodafone 4G and Three 5G for bulk downloads/backup. I've tried using only say Three 5G on my desktop but as expected, it can be quite rough during peak hours even when there appears to be plenty of bandwidth.