Is Including the Data Scheme in Your Content Security Policy Safe?
This is a great question, and I commend you taking the time to think about this from a security perspective rather than knee-jerk implement the solution from the link you sent.
Yes, as you have feared, use of data: in a CSP directive is unsafe, since this allows for XSS vulnerabilities to be opened up as data: can handle any URI. This is spelled out in Mozilla’s CSP Documentation. and in this W3C Working Draft
There is no way in CSP to specify "allow only SVG images to be embedded via data URIs, but no any other type of URIs". CSP just lets you specify data:.
As a best practice I would endeavor to address the root issue about the images being provided as base64 and see if that can be done another way so as not to require modification of the CSP directive.