I purchased a Merida Speeder 400 a year back. This came with Shimano 50-34 teeth and Shimano CSR7000 11-32 teeth 11-speed cassette which I replaced with Shimano 105 R7000 11 Speed 11-34 Teeth Cassette. Since then, the ride is not quite right. It feels like the chain is about to break sometimes especially in certain gears. What am I doing wrong?
Shimano's published method for establishing chain length is based on adding links to the length that will wrap around large/large sans derailleur. When a bigger cassette is swapped in on a chain sized that way, the chain will typically be too short, and breaking your chain, derailleur, hanger, and/or frame may result.
The alternative is to do the technically wrong thing and size to allow the RD's entire total capacity to be used, which allows any cassette the RD would otherwise be compatible with to be swapped in. (The b-tension would need to be adjusted for it to work right, but typically things won't blow up even if that step is skipped.) There are those of us who feel sending riders out with a chain sized this way is doing them better service on average than giving them whatever marginal upside there is with fewer links. This practice can really save a lot of pain. And, if you're a mechanic, you have to think about who the finger is going to be pointed at when a 25 or 27 gets swapped on when the chain was sized for a 23 and someone's race/bike/life gets messed up because of it.
It's possible what you're describing is from not adjusting the B screw, but more likely it's from the chain being too short.