If you have a Zigbee Coordinator, I’d recommend taking a look a Aqara.

If you use Zigbee the Aqara and SONOFF contact sensors are both fairly good in my experience.

The Aqara ones are very small and neat, the SONOFF are a bit larger but cheaper.

I bought my two sensors two years ago now. Haven’t had to replace the batteries yet and no connection issues or anything. I use one of them to turn on the lights at my front door when I open it and it’s always done it’s job quickly. Z-wave so ofc you’ll need a dongle.

$30 rn

https://www.amazon.com/dp/B01N5HB4U5?ref=ppx_pop_mob_ap_share

I’ve had good luck with this Zigbee one. It’s on the larger side, but takes AAA batteries which is nice. THIRDREALITY Zigbee Door/Window Sensor

Alternatively, if you’re keen to tinker a bit you can get an SDR and then use 433MHz sensors. (Requires using RTL_433 and MQTT).

I was able to use the existing sensors from my traditional security system this way. Nice thing is that it’s the most reliable wireless components on my system and the battery lasts forever.

I would suggest a nooelec SDR like this one: Nooelec RTL-SDR v5 Bundle

And something like this for sensors: Door Magnetic Contact Sensor

I have some cheap ass Tuya Zigbee sensors from AliExpress. In my experience they are reliable, quite small, i sill haven’t changed the battery, and they are super cheap. Make sure you get the Zigbee ones, not the wifi version.