Actually Maxi Cosi high backed boosters for some reason don't seem to test as well as other brands in their price range, apart from the Rodi. However, Tanza is totally new and hasn't been through independent testing yet, so might be better than the others.
But if your little one has outgrown a Group 1 seat then they will likely be too big for the harness part in the Titan anyway, so it's correct there's not much point bothering with this. If you really did want to harness then you could look at seats which harness up to 25kg, like Joie Bold. You need top tether and isofix in your car to use these seats. Personally for a 5yo unless there are behavioural issues with using a seatbelt, I'd just go for a well rated HBB :)
The top ones I'd recommend at the moment are the Cybex Solution S-fix and Z-fix - the difference between these is that the Z-fix expands widthways as well as upwards, whereas the S-fix is just upwards.
The Cybex seats have reclining headrest feature which helps with falling asleep and tend to fit petite kids well, but also go up nice and high to last you until the child is able to use a normal seatbelt. If you fit with isofix you can also recline the whole seat a little bit.
Or the Britax Kidfix seats - there are various versions of these, the differences are:
With or without SICT (extra side impact protection)
With or without XP-pad - a foam pad which goes across the child's chest and absorbs some forces of a frontal impact.
Between version 2 / 3 - the 3 is a bit more up to date and has a longer seat so more support for older children with longer legs. If you choose the XP pad version, the 3 tends to be a bit better at letting the seatbelt retract whereas version 2 has a problem with belt retraction in some cars.
All current Kidfix seats feature a "Secure guard" fourth point of contact between the legs to ensure good seatbelt positioning over the hips and help spread impact forces. And nice open belt guides at the hip so very easy to get the right placement. No recline, but a V-shaped headrest to help support the head if the child falls asleep.
Besafe izi Flex Fix is another option which comes with crotch strap to position seatbelt and a neck cushion which absorbs forces as well as being supportive if the child sleeps. It's more expensive than the other two brands.
Medium priced seats I'd recommend are
Joie Trillo LX
Joie Traver
Cybex Solution B-fix
I'll have a look for deals and come back to you :)