You do have some good points that using tileable textures wherever you can is a great way to get some good quality out of your textures.
The “Intro for Artists and Environment Art”](Intro for Artists | Live Training | Unreal Engine - YouTube) has some good tips for using your texture and adding other detail that is present in the normal as you get closer.
The key is to really find the balance that fits well with your assets and your game to give you the best quality.
We do have some options within the engine to profile where you may be experiencing any bottlenecks. You can get this by going to the viewport window > select show > Stat > select the options you want to look at.
Generally with regards to textures I try to manage how I use my textures to be most effective. I try to use the lowest size without losing the detail I would like. I try to consider what are hero assets vs standard assets. I would put much more time and detail into something that would be more visible and used by the player than a smaller object that would barely be seen by the player. Making good use of UV space on my asset is key as well! There are a lot of different things that can go into this question.
Ultimately it comes down to optimization and what works best for what you want to achieve.
Thank you!
Tim