Hi @Tyler Cope Oregon isn't like Cali (as I understand it) when it comes to assessments. Sales and transfers of title do not trigger reassessment. Land use and building permit procedures are what trigger reassessments here, so you don't have to worry about that.
The only way to know for sure if adding a joint tenancy owner would violate a due on sale clause is to see the lending documents first hand. If it's a conventional bank, it's a high probability that is a violation. However, if your plan were to refi that wouldn't matter anyway. I"m no mortgage broker so no help for you there, but it's definitely something you could look into.
There are multiple ways to potentially accomplish what you would like to do here. Thinking about tax implications is very smart as it's easy to accidentally create a tax consequence if not done correctly. Brainstorming with your CPA helps a ton with scenarios look this. If you need a local CPA, I'm happy to refer you to an excellent one locally.
As for creative ideas, there really isn't enough info to go off of. There's equity, so it can be accomplished, it's just a matter of what does that look like.
You may want to look into a life estate and see if that applies to your situation.
Good luck.