DXM gives zero withdrawal effects. Definitely none.
As for lowering tolerance, your best bet is time, time, time - as you mentioned. More time the more you're on, and more time the more breaks you've taken. I've gone down significantly with month breaks before. Also, if you do a certain amount and only "gift" yourself with slightly larger amounts, that'll keep it lower longer(what I mean by this is, say you've done 20mg per dose usually and you get a lot? well if you get a huge amount and wanna go crazy and do double for the hell of it/coz it'll feel great, just don't - maybe do 25 or 30 instead. don't get excited or anxious for more. use your head and keep tolerance in mind, and it'll make a huge difference!).
As for lowering tolerance, your best bet is time, time, time - as you mentioned. More time the more you're on, and more time the more breaks you've taken. I've gone down significantly with month breaks before. Also, if you do a certain amount and only "gift" yourself with slightly larger amounts, that'll keep it lower longer(what I mean by this is, say you've done 20mg per dose usually and you get a lot? well if you get a huge amount and wanna go crazy and do double for the hell of it/coz it'll feel great, just don't - maybe do 25 or 30 instead. don't get excited or anxious for more. use your head and keep tolerance in mind, and it'll make a huge difference!).