Sure, the image is changing now only at intentional times (as opposed to just any restart), but you're still not getting an assurance that a "trusted" upstream isn't going to RCE you. Security updates & patches come out with such frequency that the number of windows of opportunity are still plentiful.
Better advice would be to use tags, but to vendor the images somewhere and point at the vendored copies. You still get the "doesn't change on random restart", you have an idea of what version you're on that the hash doesn't tell you, you don't have to deal with Docker hashes being the weird AF things they are (e.g., you side-step the confusion that library/foo@sha256:XXXXX == vendored/foo@sha256:YYYYY), and you're more respectful of upstream's bandwidth and you're not dependent on their reliability concerns.
(And frankly, given how poorly most of the industry handles security updates, I'd argue that sitting on a very loose tag might be more secure overall, because then security patches might be happening at random times, as opposed to not at all. The trade-off of the RCE is worth it, and there are ways for upstreams to mitigate the risk there, centrally. Most companies struggle with just vendoring, and few get to actually keeping that up-to-date, IME due to a lack of willingness to apply the necessary resources to the problem.)