Yes, again read and try to follow this → Zigbee networks: how to guide for avoiding interference and optimize for getting better range + coverage
Note there that you may need to re-pair some old devices if you have Zigbee devices with bad firmware, otherwise, all Zigbee devices with properly written firmware should automatically move to a better route after around 24 hours or so. You can sometimes force battery-powered devices to connect to a better route by power cycling them (again if they have good firmware).
Indeed. My recommend is to only use a Zigbee lightbulbs in locations where you have removed the physical switch to it so that no one can/will accidentally turn it off and on, (because Zigbee Router devices are expected to always have power so other devices connected to them will take time to move and thus become unavailable for a while). Or alternativly only buy Zigbee lightbulb models that are purposly designed to not act as Zigbee Router devices (i.e. they only act as a Zigbee End Device, also known as ZED in the Zigbee network).
Personally, I suggest instead using dumb lightbulbs using smart Zigbee switches where you replace the whole switch or use a switch module to convert an existing switch into a smart switch. Zigbee lightbulbs that do act as Zigbee Router devices can still have their place in example loose/movable window lights and floor lights with cables where you can more easily remove the switch from their cable.
Also, before buying any new Zigbee products, note that not all mains-powered devices are Zigbee Router devices, and note that some Zigbee Router devices are much better at acting as a Zigbee Router device than others. It is a good idea to research that specific Zigbee product before buying so know if it will also be a good Zigbee Router or not if want more routers, otherwise best is to just buy a few dedicated Zigbee Router devices.