I have an issue with buffing the champions and not Sunfire

Namely that this would just make the item even less appealing on them. If Sunfire is supposed to be the item that you build when you want to be able to push waves, either on a tank with bad waveclear or just to spend less mana to waveclear, giving tanks that ability to push innately in their kit will make the item completely worthless compared to as if it wasn't already always the better choice. Cinderhulk is also quite weak, but I suspect that could be fixed just by raising the % bonus.

Secondly I have the suspicion that giving tanks the ability to waveclear naturally will make them gravitate towards damage/tank oriented items like Titanic, Sterak's, and the AP counterparts etc. This is already what I've been doing with Sejuani top lane; because I have strong waveclear/pushing after level 7, and because Bami's and its upgrades aren't worth the gold, I almost always invest in damage items first / . I think this will be the case for other tanks who are currently underperforming post-Sunfire nerfs once it catches on.

Buffing Sunfire would just be the easiest option, and IMO it makes the most sense if you want tanks to build like tanks.

Editing context https://boards.na.leagueoflegends.com/en/c/developer-corner/KTswL4uY-quick-gameplay-thoughts-may-9

Sunfire Rushing Tanks - We're seeing a number of tanks who'd previously been in reasonable spots balance wise struggle a bit more after the Sunfire changes. That's not unexpected, indicating they had problems laning that Sunfire was covering up. Likely direction there is to adjust individual kits so they function more appropriately, rather than using Sunfire as a generic solve.