You should consider a one time, lifetime payment. As a solo dev working on occasional side projects I just wouldn't even consider something on a subscription, and $140 (balsamiq's one time fee) is about $100 more than I'd pay. My alternative is a graphics app I already own.

Follow what Affinity did (cheap and one-time) and you'll sell to a lot of people like me who would otherwise give it a miss. Save your subscription tiers for businesses needing more collaboration, SSO, etc.

With that strategy as well you'll build brand awareness which will probably ultimately lead to more sales as those solo devs advocate for its use in teams in their day jobs.