Donations to the wat : indeed, that's a concern. My partner is more sensible than most others I see around in her region, and she has seen the world with me (outside Thailand) so she has a bit of a sense of reality. However, she remains a Thai person, and whenever we are back in her village, she turns back being glued to the wat and everything that revolves around.
Buddhism is a nice thing, which I respect, in principle, but the way it is practised in Thailand is a corruption of what it origially was meant to be....
Pension: yes, I have pensions (plural), related to past employment. However, not being legally married with my Thai partner, and still having a legal wife in Europe (very difficult to divorce for various reasons), the law in my case - given my nationality and where the pensions are being paid - would completely override any will I might have signed, wherever.
So, a reasonable option I get from a number of contributors in this discussion would be to invest in a condo, on my Thai partner's name, that would secure her a regular income. Key would then be to invest in a quality condo with quality management. And hoping she would not sell it once I'm deceased to waste it all between the wat and her family, but again, that would be up to her. There are limits to what one can do!