I wouldn't put him in a high backed booster if you have the option of a harnessed seat. If you absolutely have to, then Chicco Fold and Go is the best travel option, but a 16kg 3 year old is not well protected in any high backed booster. Doubly so if the seatbelt is dubious or only a lap belt. Also he is technically not tall enough for it - he should be 100cm. A very high quality harnessed seat will likely offer better protection, even if the seatbelt is not ideal. Rear facing would be excellent, but I think you'll struggle to fit most rear facing seats so that might be a no go especially as he'll be quite tall and heavy in the shell.
It's worth familiarising yourself with how American seatbelts lock for child seat installation. This is not something that you should do if the seatbelt inertia lock (locks when you yank it) is working properly, because European car seats are designed to work with European style seatbelts that don't have this locking mode, but in the case that the only seatbelts that you have access to are dodgy, it might be something to bear in mind.
Out of what you've suggested, I'd go for Romer King - this is a good, well rated seat with a belt tensioner and Britax-Romer build in a decent amount of "failsafe" into their seats which is a decent thing to have if you're unsure about availability of seatbelts etc.
I don't like Joie Fortifi, this is very similar to the Joie Every Stage, which did poorly forward facing, and I would avoid 123 type seats if possible, stick with something with a solid one-piece shell. Maxi Cosi Nomad is not that dissimilar from Urban Kanga. It probably has better side impact protection. Did you find that you were able to fit the Urban Kanga securely in most vehicles? It always looks to me like it would get forward anchorage (where the belt is taut sort of in front of the seat, so the seat is effectively loose behind the taut belt. This is wrong, as the belt should be "anchoring" the seat by pulling it back). Maxi Cosi has the same problem because the back of the seat is very thin.
One problem you'll struggle with in Germany is that since September of this year, R44 regulation seats are only allowed to be sold as far as existing stock lasts. All new stock needs to be R129, so you'll find dwindling stock of a lot of these products. There are very few R129 belt fitted, forward facing seats on the market thus far.
So, normal, R44 Group 1 seats with decent(ish) safety rating up to €300 which are still sold (that I can find for sale - I'm also in Germany 🙂🥨)
Maxi Cosi Axiss - 9kg, good safety rating, the shoulder belt routing is pretty crucial.
Maxi Cosi Priori - the previous model had lap belt approval. This one officially doesn't. Very lightweight. Used to be the go to model for non drivers. A bit basic/outdated.
Maxi Cosi Tobi - one of the best rated seatbelt fit seats. However again the shoulder belt routing is crucial.
Maxi Cosi Beryl - Great because of the long harness use. But it's a bit of a beast at 14.9kg. It does come apart into seat and base (which can be belt fitted).
Britax-Romer Duo Plus - this would be a great option because it has both seatbelt and isofix/top tether fitting, either can be used. Gets a good safety rating in both settings (better with isofix). It's heavier than the King. You can use isofix fittings in LATCH anchors. Official fitting does not allow use of tether with seatbelt, but worth noting that several US seats fit this way, if you are ever in a non ideal fitting situation, it is worth knowing.
Britax Romer Eclipse - very lightweight, has lap belt approval, has plane approval. AutoMotorSport were quite critical of its performance in the lap belt test back in about 2013.
Britax-Romer King II - one of the best rated seatbelt fit seats.
Recaro Tian - 8.4kg - it's a 123 seat but probably the best safety rating of all of these/most modern one. It has isofix, but you only use it in the booster mode. Harness mode is belt fitted.
Besafe izi Comfort X3 - this is a beast weight wise again but they are good seats, great safety for forward facing. Might be impractical.
You can get Joie Stages/Tilt/Steadi R129 or Graco Extend / Extend R129 (all for basically extortionate prices) - these are all fairly lightweight and the R129 ones have the extra testing of R129, plus might be more easily found. They all have the option to rear face up to 18kg, which might be worth looking at, but the forward facing modes on these are pretty basic, similar to the Britax Eclipse/Maxi Cosi Priori.
Also Joie Every Stage R129 / Graco Slimfit R129 is an option for rear facing up to 22kg/105cm. Also has forward facing mode but only without harness (high backed booster) so could be a bit risky for shorter seatbelts.
In general, the brands I'd trust for forward facing in Germany are Maxi Cosi, Britax-Romer, Besafe (HTS), and Recaro. If you can go and scour any local shops including supermarkets, for any old stock that isn't listed online. Ask at your nearest Zwergperten or Kindersitzprofis, BabyOne, BabyWalz. They might even have some retired rental seats that you could offer to replace with an R129 model. It's worth an ask.
If you are comfortable with buying second hand, Diono Radian 5 used to be suggested a lot for travel because it folds and it's flight approved. But it's tricky to use with very complicated instructions so if you do buy it second hand, be sure to get all the parts and the instruction book.
There are some impact shield seats and Germans are very keen on the safety credentials of these but I'm not sure I'd go for one in this situation, as they are meant to perform worse in rollovers and offset crashes. However, they are very easy to fit quickly in pretty much any sort of car, and the Kiddy ones have lap belt approval. (I know that Joie and Cybex don't - Recaro IS might). At 95cm tall, this might be worth considering. Kiddy Phoenixfix seems to still be sold.
Bit of a wildcard suggestion - but Nachfolger Hy5 World?? This is a self-inflatable seat, rear facing, belt routing shouldn't be a problem. Zwergperten stock them in store (or they used to). It is about €400 though so it is over your budget but the practicality might sway that. It can also be used on a plane.
Another wildcard suggestion would be to see if you can import a Ride Safer Travel Vest from America - though in all honesty, I am really not sure if I would trust it. There are some positive stories on their website from people who have been in accidents, so maybe just one to research.
Definitely take something with you rather than buying out there - the car seat selection in Central America is not good at all. And according to a friend who travelled there, don't leave it visible in the car - it is apparently taken as a sign that you're "rich" and there might be things worth stealing in the car.