Re: Pretty sure 3G and 4G can work at the same time...
Not yet, but eventually.
5G latency improvements don't come until Stand-Alone 5G is live (maybe 2023~24 in the UK). At the moment 5G latency is limited by its signalling running over 4G.
At the radio level there's minimal power difference between 4G and 5G. Spectral efficiency is also about the same.
Right now user density would be much higher for 4G2100 than 5G2100 simply because there are 10x more 4G2100 devices out there. That will probably reverse around the same time Stand Alone arrives. The full density capabilities of 5G aren't needed for handsets though; it's more about Internet of Things.
2G interleaving is only an option in the 900 & 1800 bands in the UK - no 2G2100 devices exist. Unlikely to see 5G in 2100 just yet though; there are plenty of other bands available that require less work. For example 700 & 800 give much better coverage than 2100, while 1800 would be a better option for capacity.