Home assistant is a beast, not sure openhab can actually match it today. Unless you already have a pi5 and a SSD (avoid micro sd card), you could consider a mini pc with n150 (or n100) processor. It doesn’t consume much power (close to pi5) and it will be more powerful and more Ram which will be useful for immich.
Start with zigbee first, much more mature than thread. I don’t personaly use thread but i believe some zigbee dongle also can deal with thread at the same time.
About the setup. If mini pc, you can use proxmox so you will be able to create a VM for home assistant and another one for immich.
A pi5 with 1gb ram will always be cheaper but that’s not enough ram for your usecase, search n150 at Aliexpress, you can often find some mini pc for around 150-160€ on sale with n150, 12gb lpddr5 (or 16gb ddr4 sodim) and 500gb SSD. https://a.aliexpress.com/_EQ5k2Uu This one was 153€ couple of days ago. Thay are other Chinese brand like soyo, beelink, bmax that have similar hardware. They are not as good as NUC, but they do the job for way cheaper.