Good of you to ask, and I believe the Thai OSM mappers are a little divided on this one…
I recently had a discussion with a mapper who decided the best way to make vehicles go around Nong Bua Lam Phu, was to down grade the 4 lane Primary Hwy 210 to tertiary, while upgrading the Rural route 3038 (the bypass), to Primary. I argued that changing road status to force navigation a certain way was not correct, and against our Wiki advice.
This also brings another issue into focus where it is my belief that the tagging/status of a road should follow its Government designation. Ie, 2 digit highways =trunk, 3 digit=primary, 4 digit=secondary, and 4-digit with a province prefix, (known as a Rural road) should = tertiary. This is documented in our page https://wiki.openstreetmap.org/wiki/WikiProject_Thailand#Highway_classification
Now I realise that in a few places, predominantly towards Bangkok and the South, we do have instances of roads that have been mapped according to the physical characteristics of the highway. For example, a 3 digit highway might be primary where it is 2 lane, but becomes trunk where there is a four lane dual carriageway section. It is my belief that this is not good mapping, but I tend to respect others work and until we agree unanimously, I leave as is.
One clear disadvantage is when the maps are viewed at certain zoom levels, roads can appear very broken, until you zoom in. I would welcome comment from other seasoned mappers on this argument.
So back to your dilemma … to avoid being routed through a city, firstly you can check to see if a “big wide road” in the city has been tagged as trunk or primary. If it is not part of the official Highway, and does not carry a route number, then reduce its status to something less.
Another key tool, is to add the maxspeed= tag, to the city sections …which in fairness, can be taken from road signs, but I always think that if that info is not visible, then you are able to tag with the Thai legal speed limit in built up areas. The Thai rules are very confusing but if you used 45 kmph, that applies to most cars & trucks, so would work for your needs, without being too pedantic.
In the UK, I see they do this, and just add source=NSL (National Speed limit).
Throw in any Traffic Lights you know about, and it just may help … but remember, Navigation is just a calculation, and one local guy I know kept ranting about poor routing in Chiang Mai, sending him down the small Soi’s etc, etc. One look at his Garmin confirmed it was set to “shortest distance”… my point… do whats right for the map, and not try to make it influence the GPS.