New In HA!Need advice on what hardware to invest

Hello to the HA community this past month since my house is getting renovated I ‘v been itching to get into smart home things.After a months of research I have realised that the most secure /internet free way of controlling everything is HA.I installed my Hassos on my home server through proxxmox everything works fine no problems with installations .Right now I only have 4 sonoff switches and one echo dot.I manged to install sonoff lan intergration and I can control the switches through ha app.All good .Now to the main point sorry for the intro . My main goal is to have an internet free home meaning in the event of internet outage I want everything to work . But some things like boiler for shower or maybe thermostat I want those to be able to be remote controlled .I understand that best thing is to invest in a zigbee gateway and then build a zigbee mesh .But what zigbee gateway? Does it need to be flashed to integrate with ha will it be able to control different brands .Should I just buy only WiFi products?But they must be flashed with tasmota if am not mistaken .To give you an idea I would like my home to have 3-5 Door window sensors.temp senson in every room .all lighgts to be smart .video doorbell .and also thermostat .I don’t want Chinese servers to be able to control my staff or get my info but I also want to give Alexa voice commands to turn on light for exAmple…Sorry for the long text first time posting on ha forums .If anyone has any advice I would be thrilled to discuss with someone knowledgeable meanwhile I’ll keep reading guides thank you

Any zigbee stick supported by ZHA or Zigbee2MQTT will do. They don’t need any cloud servers.