German Brewing

Yesterday I brewed my first LoDO beer, a Weissbier. I don't have a DO meter yet, but I followed the steps outlined in the Helles pdf. I oxygenated the wort like I usually do (except this time I pitched first), using a 2 micron stone with pure O2 for 60 seconds. There was a lot of foam created (as usual) and then I noticed a smell of burnt matches. A web search turned up that sulfite reacting with O2 makes SO2, which can smell like burnt matches.

Did I oxygenate too quickly?

After doing some more reading on this forum, I think I should be oxygenating slower (even if it's not the cause of the burnt match smell). Any ideas for how to do this without a DO meter? (I know, I know, I really should get one).

I didn't smell any sulfur in the mash or in the post-boil wort before oxygenating.

Get a DO meter. Especially when using pure O2

I have an aquarium air pump that I can use instead of pure oxygen. I'll pinch the hose so that I get just a trickle of bubbles through a 2 micron stone and I'll let it aerate overnight. Does that sound reasonable, or can anyone suggest a different recipe?

Re-pitching lager yeasts will always increase the sulphur production. But whether this specifically is the cause
I don't know.