Option 4) - adopt a risk-based approach to compliance, and look to assess whether any aspect of your service, and the way it makes use of data in its current form is an egregious breach of GDPR. If that is the case, you're likely in breach of existing data protection law.
In terms of risk factors, most side projects will generally use data to provide a service to customers. This type of processing is unlikely to attract regulatory attention.
It's also unlikely that the vast majority of entities will be 100% compliant come May 25, but as the Belgian data protection authority has pointed out, it is important that people demonstrate a good faith approach to compliance.
For new requirements around restriction of processing/provision of data in a programmatic manner, again, dependent on risk, it is likely not necessary to implement these features and side project owners should focus on building their product instead.
Relevant risks for a side-project owner are around a) volume of data held, b) types of uses of data, c) likelihood of users making a requests around erasure/restriction (look at any historic requests received here), and d) regulatory focus on specific areas of the legislation.
This law is not going to fizzle out and on a general level, in my view it is advisable for any entity to look to respect user data regardless of legal obligations relating to that.