At 16 months they really should be rear facing anyway. If you are really not keen on RF for the long term, you could get a cheaper seat for now which will rear face up to 105cm/18kg ish and then look to change to a longer lasting seat and make the decision at that point about RF/FF, based on how your child tolerates RF (bearing in mind the longer lasting seats have more space and are less bulky in the car, due to being more upright), and where they are on the height/weight centiles at this time.
For example, Graco Turn2Me or Joie 360 Spin are around £110, Joie Stages £85, Graco Extend £75. Each are suitable up to 105cm (about 115cm for Stages) and 18kg.
The longest lasting forward facing harness seat (without counting special needs restraints) is the older Joie Bold R, which goes up to 25kg on the harness and unlimited height as long as the child can fit under the shoulder straps (which go up very tall, easily 140cm+).
There is a newer regulation version, Joie i-Irvana max which goes up to 125cm / 25kg on the harness.
I wouldn't be keen on a 16mo in either seat, they don't really recline and they aren't very supportive for such a young child, also they are only forward facing.
The longest lasting rear facing seats go up to 125cm (but some can fit longer) and 36kg weight limit, so they do last significantly longer which can make a difference for children on the highest weight lines. These are Axkid Minikid (various versions) and Britax Max-Safe Pro and Britax Safe-Way M. Legally these seats max out at 125cm but I have seen people suggest that using the Axkid in particular to ~130cm is no issue. (It would be up to you if you wanted to use it outside of the legal standard like that.)
You may also look at Besafe Beyond 360, although it's very expensive as needs a separate base, and may not last long enough if baby stays on the same centiles. The limits on this seat for both RF and FF are 125cm and 21kg. I would not exceed them due to it being a base + seat combo. This might not last you long enough for weight.
If you look on the centile charts which are here: (With a link to 2-18 years at the side)
https://www.rcpch.ac.uk/resources/uk-who-growth-charts-0-4-years
And follow your child's centile line roughly up, you can see what age they will roughly be when they reach 125-130cm and then 21kg / 25kg / 36kg. Centiles can change and some children drop centiles when they start to walk/run, so this is only ever an estimate, but gives you an idea.
You don't want a child in a high back booster using adult seatbelt until they are at least 4 really. Never before 3.5, but over 4 is much better.