I have some experience in fundraising, and while it's a big topic, if I might suggest starting out with putting out a release announcement on your blog for each update (including betas) that also reminds people you're working towards a goal of working on k9 full time. Add a progress bar and donation link below. Then you link to each release on social and mention your social accounts on first run of the app and on the About page, to get people to those release announcements.
The social network of the future: No ads, no corporate surveillance, ethical design, and decentralization! Own your data with Mastodon!