I've been using my 6T paired with a Fitbit Versa for the past 7/8 months...so far so good. No drastic impact on batterly Life, I still manage to get 6 hrs SoT on medium usage (social media, web browsing, light gaming) on stock rom.
I'm currently using MiBand 4 as smartwatch replacement, although it's not a smartwatch, it gets everything i need done, like working out, get notification, control music player, find my phone, etc. And it's very cheap, light, and durable
In the last month, I used the Samsung Galaxy Active 2, before this I used for two years a Huawei Watch 2. Best experience with Huawei. The Samsung, even if it is a best buy, is too much limited by the non-presence of the Samsung infrastructure in the phone (One UI at all): music application, alarm clock, automatic do not disturb and other little features doesn't communicate with stock android apps of the OP6T. So, I returned it and bought a Fossil Gen 5... Waiting it!
Huawei Watch GT, bought mine 2nd hand for only €30 and it's been working great so far. Although it's more of a fitness band on steroids than it is a full fledged smartwatch.
Also a fitness band, but the Xiaomi Mi Band 4 which I had before the Watch GT also works great and is around the same price range.
I'm using the Galaxy S3 Frontier, which pretty routinely goes on sale. Right now it's on sale for $230 pretty much everywhere. It will do pretty much everything a phone will, even has built in GPS for stand alone GPS if you want. Note the Frontier has the GPS the regular newer Galaxy model does not.
I bought the Galaxy Watch Active and it pairs nicely with the OnePlus6T. I was able to receive all notifications. It's super light weight and I even wear it to bed. However, it uses tizen 4.0 with one UI and it's only $200.
Previously was a huge fan of Wear OS (Android Wear) but my Zenwatch 3 had gotten so slow. I upgraded to the Galaxy watch 46mm. This watch is so snappy and Samsung has allowed it to be used with more than just Samsung phones.
As a side note, I got Samsung Pay working with my rooted 6T. I can send anyone the instructions if they need.