> I do like the swivel base but feel like the room the take up is crazy!
This is not the case so much for the next stage. The current seat you have is very reclined, because it is designed for young babies. The i-Level in particular is a very large seat. Any seat which swivels for the next stage, will have a recline setting that can be used for younger babies, but can also be sat more upright e.g. if your child is unlikely to nap, and then they take up less room. In my experience, seats which are a separate seat + base (and can take toddler seats too) tend to be even bigger in order to make space for everything. Something which is only for one seat and the seat doesn't come off the base, they can be optimised to take up much less room. So the Cybex Sirona Gi i-size as mentioned, or Joie i-Pivot, or Axkid Spinkid, these would take up less space than your i-Level seat on the swivel base.
> But if I go for isofix only then its rear facing and eventually after 14 months or so will have to then buy another seat that then faces forward
No this is not the case - the vast majority of isofix seats on the market for this age group are swivel seats, which can be fitted both forward and rear facing. They act exactly like your seat does when it's attached to the base, it's just that they don't come off the base.
Some are rear facing only, like the Axkid Spinkid. But most of them (Joie, Graco, Britax, Maxi Cosi, Cybex) can go either way. It's also possible (and safer!) to keep your child rear facing for the whole of this stage, that's an option if you wanted to. But if you're sure you'll want to forward face at some point, then a spin seat gives you the option to do that. If your baby is 10 months now, then 14 months in the future is about age 2. It's better to wait until age 2 than turn her immediately at 15 months old, but it's even better to wait until age 3 or even 4 to turn her forward, and there are seats designed for this if that's what you choose. You can also do this using the 360 spin swivel seats although the leg room tends to be a bit less.
It's only the longer lasting rear facing seats, like Axkid Minikid, which are rear facing only. Most of those are not isofix, they are fitted using seatbelt and tethers. The benefit of these seats is that they sit more upright, so take up much less space, they can be adjusted to have a lot more leg room for older children (designed in Sweden where they advise rear facing until age 4 - so think about tall, long legged Swedish children!) and they have often passed the Swedish Plus Test which is a mark of safety a lot of people like.
> Saying that this current seat goes up to 18 months then? Or 105 cm then which ever comes first.
Your current seat which you have (Joie i-Level) has a height limit of 85cm (not 105cm - that is the limit of most of the spinning/swivel seats often sold as "up to age 4") and a weight limit of 13kg, it also has a headrest which can move up and down.
Babies or toddlers are too big to use the i-Level when ONE of the following happens:
They are over 85cm in length/height
They are over 13kg
The headrest is on the highest setting, and their shoulders no longer fit underneath this.
It does not have an age limit; 18 months is a rough idea of when she might outgrow it, based on average sized babies. Your baby might be taller than average and so she might reach one of those limits earlier, or she might be smaller than average and be able to fit into the seat safely for longer. As long as she is within ALL three of the above limits, she can use the seat, legally and safely. It doesn't matter if her legs are over the end, or if she is older than 18 months. These are not safety concerns 
> this could last til then then go from this to forward facing when little baby arrives
If you keep her in the Joie i-Level until she is over 15 months and then buy a new seat for her, it would be legal to buy a forward facing seat, yes. But personally, I would not recommend this - 15 months is so little to be forward facing, and there are plenty of seats on the market which offer rear facing for longer plus the ability to turn forward when you're ready, which also have the swivel function which you like - this is especially helpful for toddlers!
Most of the forward facing only seats on the market these days are very large and really designed for children aged about 2.5+, they don't swivel, and the recline function is not very good. IMO, it's slightly better to FF in a seat which is designed and optimised for FF, than to FF in a spin seat which can also RF. But the benefit of keeping her RF until she is 2, 3, or even 4 is more important, so I think a spin seat would be the right choice for you.
The main thing I was saying to avoid is the "All stages" seats, which often say they go up to age 7 or age 10/12 because they also include a high back booster stage. For example, Joie i-Pivot Grow, or Britax Swivel. These are bigger in the car, less recline and you don't need to combine the stages if you want to have another child. They also get less good scores for safety.
Hope that helps and feel free to ask any more questions.