Comments
Alex Foster's answer covers most of what I wanted to say, but I'll also note that thinking in this way...
On the flip side, being in a relationship might help cut costs enabling more donating and could increase happiness to have an indirect effect on productivity?
...is unlikely to be conducive to a successful romantic relationship, unless this thinking is secondary to thoughts like "I really like this person, they make me happy, and I also want them to be happy".
(There's a word for a relationship you form to cut costs, and the word is "roommate". There's a word for a relationship you form to increase happiness, and the word is "friend". As a bonus, it's easier to have multiple roommates or friends than it is to have multiple romantic partners.)
If you want to try a work strategy that involves long hours then a positive successful relationship may be harder to achieve.
Otherwise I’d advocate you don’t instrumentalise your non-work time for impact. I know it’s a cliche but do what you enjoy. Instrumentalising your free time seems to make people less relatable (probably an understatement), less trustworthy, more prone to depression, less robust to sudden changes etc
Having a strong base, whatever it is for you, is pretty important I think. When impact stuff is going badly you don’t want to feel like everything is going badly. That kind of instability is going to have more Long term effect for your impact than a few thousand pounds a year in one direction or another.