I can also answer from my perspective, as someone who worked in platform and now works in product…
1. Product is not engineering. PM skills are hard to learn - Engineers trying to learn product without a good internal mentor is going to be exceptionally hard. If your environment doesn’t have good product role models already, converting a platform engineering into a PM is going to be very hard. They might not be successful, and you might burn that platform engineer or the platform initiative itself by putting too much pressure / expectation.
2. Promoting a platform engineer directly is unlikely to produce “outsized” results - I would be deeply, deeply surprised if you can promote and engineer to a PM title, without any serious mentorship, and then get significantly outsized results, beyond let’s say just having a bunch of product-minded engineers in the first place. That engineer is going to do their best, but underneath it all, they are still an engineer, and I believe they might risk falling back on trying to use “what made them good” at engineering, but trying to apply that to product. See point #1.
3. Hiring outside PM’s with lower tech knowledge could help, but mostly for coaching - A solution could be hiring an outside PM with or without deep tech knowledge (doesn’t matter, aslong as they’re a good PM). Or, bringing an internal PM “on loan” from a product team to support and guide. Having a platform candidate then step up and be mentored by a seasoned PM seems like the best middle ground. Platform and existing tech knowledge, twinned with seasoned experienced PM input.
If I were starting a platform team again from scratch today, I’d go with #3 if I could.