I used to work in a car seat shop. The combination up to 12 years seats are extremely popular, because most people have the same assumption as you. So - yes - many people buy them.
Unfortunately I think the assumption is wrong! In general I would always advise getting a separate seat for each stage. The different car seat stages have to perform very different jobs, and it is easier to build seats which are optimised for one (maybe max 2) jobs rather than trying to compromise across more roles. The more "things" it does the more compromises it has to make.
I tend to find that toddler seats that go on a base like baby seats are very expensive. Looking at the Joie i-Harbour it's £280. There are plenty of toddler seats (including with spin and isofix) for around that price or in fact much less. Joie have spin seats starting at £200. You can get a decent, seatbelt fitted toddler car seat starting from about £100. You can even get extended rear facing car seats which last much longer than the spin ones, starting at £200. I would forget the i-Harbour, which will just take your base out of use, and look at other options.
The other problem is if you buy a toddler seat that goes onto your i-Level base and then you have another baby, you can't have both seats on the base at once, and neither of them can be fitted without the base. So personally I would just decide about the second base in relation to your infant carrier and not worry about whether it takes a toddler seat. With the i-Level Recline, you do also have the choice of either the spinning one which perhaps you have as your main base, or the i-Base LX2 which is about £50 cheaper.
Another option might be if you are sure you will want a spin seat for the next stage, you could just get that straight away and put it in DH's car. They are not always as well fitting straight from birth, but from a few weeks they are just as good as the infant carrier and simply slightly less convenient. Or purchase a second, cheaper infant carrier for DH's car which can be fitted with seatbelt.
With the up to 12 years car seats, there are two types - the forward facing only type, which are sold as suitable from 15 months (or sometimes from 9 months) - I wouldn't recommend these as a straight follow on from the infant carrier, although they can be a useful option for children who are growing out of their toddler seat, or vacating it for a younger sibling, but still need the 5 point harness for a bit longer. These are better from about 2.5 years. I would not personally want a 1 year old in one, they should be rear facing at that age.
Or you have lots of seats on the market now which claim to do all the stages from birth right to 12 years. These are almost always a false economy, awkward to use (misuse is a serious safety problem for car seats), very bulky, they often don't fit well at any of the stages compared with a seat that just covers one stage, and they typically score worse on crash safety testing compared with seats that only cover one stage. This is true even for premium brands where their seats tend to perform well in crash testing, such as Nuna. Recently, Silver Cross had their all-stages seat tested in one of these crash tests and it caused a bit of a stir because it did better than most all-stages seats tend to do. However, it still has a worse score than the spin seats up to ~4 years tend to get, and it costs about £400, so it's not exactly money saving! There are some very cheap all-stages seats for sale online around the £150 mark. Personally I would not touch them. They are cheap for a reason.
Booster seats are not as expensive as car seats at earlier stages - you can get a really good one for less than £100 and a decent but basic one for about £50. It's worth investing your money into the toddler stage as this is where it's most likely to make a difference.