Chiapas was never part of Guatemala, except for the Los Altos region you mentioned above. It was a separate part of the Captaincy General of Guatemala upon independence from Spain. When Central America decided to become independence from Mexico and try their brief federal experiement, Chiapas decided to stay part of Mexico. Guatemala was considered of having too much electoral power in the new United Provinces of Central America (UPCA) and was broken up into Guatemala and Los Altos to be fairer. But as the federation collapsed and all the states proclaimed independence, Guatemala invaded Los Altos to take it back. At the same time, Mexico took the Soconusco region of Los Altos and added it to Chiapas.
A Guatemala with Soconusco probably would not be too much different than OTL. It is an extra piece of coastline but doesn't remove Chiapas's coast. Maybe it would actually have been worse for Guatemala if keeping Soconusco leads to border disputes and possibly war with Mexico. Where did you read that it was pretty wealthy compared to the rest of the surrounding regions? I read that Central America, including Chiapas, was relatively resource poor and considered a political and economic backwater for much of its history up to independence.
An interesting timeline would be if Chiapas decided to become independent and how it affects everything in Mexican and Central American history. And I am always in favor of more UPCA stays united timelines as that would have a major impact on world history, but it is hard to find many Central American experts here and hard to find thorough books on Central American history in physical bookstores. I had to take some risks and find some decent ones online. My wife is from Honduras so I am interested in that part of the world's history now. The various wikipedia articles on those topics are fairly decent though.