If you're fine with a used phone, the OnePlus 8 has a high-end Snapdragon 865 processor and 8 GB RAM.[1] The carrier models have IP68, and unlocked models are manufactured similarly but don't have an official IP rating.[2] If you're getting the T-Mobile carrier model (which may be carrier unlocked at sale), you'll need to request a code and wait a week to unlock the bootloader before you can flash LineageOS.[3] Used models go for $200-300 on eBay depending on condition, and a new factory unlocked model is $399.
If you're looking for a new phone, you may want to consider the Pixel 5a which manages to have both IP67 and a headphone jack for $449 new, but uses a mid-level Snapdragon 765G processor paired with 6 GB RAM.[4] The OnePlus 9 Pro is also available with a high-end Snapdragon 888 processor, 12 GB RAM, and IP68 for $969 new or about $600-800 used.[5]
[2] https://9to5google.com/2020/04/14/oneplus-8-ip68-water-resis...
[3] https://www.oneplus.com/support/answer/detail/op588