How can I prepare a heavy 3D scene for real-time performance?

working on a large mixed-use project at the moment and the model has gotten to a point where real-time performance is starting to suffer. not unusable but noticeable enough that walking a client through it live feels risky

i've done the obvious stuff, purging unused assets, swapping out some of the heavier vegetation for proxies, turning off layers that aren't visible in the walkthrough. helped but not enough

what i'm still struggling with is knowing where to draw the line on geometry detail. like how aggressive do you actually need to be with polygon reduction on furniture and objects that are in the scene but not the focal point. and does it matter what renderer you're in, or are the optimisation principles basically the same across real-time tools

also curious about texture management because i have a feeling that's where a lot of my overhead is coming from but i haven't done a proper audit yet. is there a sensible threshold for texture resolution on background vs foreground objects or is it more situational than that

would love to hear from anyone who's had to take a genuinely heavy scene and get it running smoothly for a live client presentation. what's your actual process and what tools do you guys use?