Assuming the reason is a true stall, occurring more than a year after starting, or low rates of weight loss with a degree of hunger that is difficult to be comfortable with, it is reasonable.
I think it depends a bit on how severe the obesity is in the first place, but most of the time it has to be a pretty major problem if 15mg of tirz was not enough. But in general the obesity needs to have been severe enough to justify unknown extra risks from high dose/combo strategies.
The only higher dose studies so far were for semaglutide at 7.2mg and 16mg, both caused some extra weight loss, but not huge amounts, and had a lot more side effects. But importantly there were no new severe unexpected adverse effects from the higher doses, while this does not prove higher dose tirz is safe it is at least a bit reassuring.
Going from 15mg up to 20mg probably depends on what happened when you increased from 10 to 15mg, if you found those increases helpful, and did not cause major issues with side effects , then increasing the dose might be reasonable.
Otherwise adding in elora or cagri makes sense, but at low doses with slow increases like 0.25 of cagri or 1mg of elora. Cagri is cheaper , elora is super new and less is known but is probably more effective with less side effects , especially at low doses of 3mg or less, but at higher doses above 6mg does also have high rates of nausea and fatigue up to about 50/50%, but a bit better than high dose cagri.