I’m currently in the process of filling 50 reclaimed white oak stair treads. I’ve found light passes in the planer is okay, but go too hard and it pulls some out. Im using west systems with the thickener.
Light passes to knock it mostly down and finishing with the drum sander is doing the trick.
When I fill to a smaller scale I usually just use the LN 102 or 15mm kote nomi. The little 12v bosch electric plane works well too. So basically anything that is gentle enough to not rip it out. Which is most tools I guess.