There are two versions of the Titan, the older one is the Titan, Titan pro, titan plus or Titan i-size. The harness is small on this and it doesn't fit very long so your LO might already be too big for it. Check it in a shop rather than buying online.
The newer version is the Titan pro i-size or Titan plus i-size (it is a bit confusing with the names being so similar) the shell of this one is bigger so logically the harness should be as well, but I don't know for sure. The limit on all of the versions is 18kg so you'll need to check your LO is under that too. The crash test is better on the newer version.
Joie Bold has the harness up to 25kg instead so a good choice if you need the extended harness, but wouldn't be my pick otherwise as the crash test isn't that great. I'm not a big fan of it as a booster seat. I think it will feel like a step down in quality after maxi Cosi.
I have the advansafix for my 4yo and am happy with it. He fit into the harness until approx 109cm, it's a good bit bigger than the old Titan model. I liked that it has the crotch guard thing and turns into a well rated booster. The i-size version technically has a 102cm limit on the harness for reasons unknown. He's never complained about the padding. We've done a long drive in it (10 hours) x4 and he seemed comfy.
Britax do have the new Evolvafix which is similar to the advansafix but with a 105cm harness limit.
These brands are fairly widely available so you may be able to try them out which is always worth doing.
Maxi Cosi kore is nice too if you wanted just a high backed booster. The only thing is it doesn't go up quite as high as some other brands and the ADAC test result was not quite as good as for some other brands (but it was still ok).