There's no evidence to support the idea that FF in a 5 point harness is somehow dangerous, unless you're talking about tiny babies. Yes rear facing is safer, but it's not the only safe option. Most of the "never ever FF in a harness" stuff came from a car seat company trying to market impact shields, apparently, and it's never really gone away. It's certainly not something you should avoid out of fear, though it is perfectly valid to skip it as a stage if it's working for you.
At 3.5 it's better to be in a harness than in a HBB - it just is. This is really clear. Where it's less clear is for older children ie over 4, over 18kg - there, we don't have good evidence either way to say high back booster vs harness is better. Both have good results to protect children in car crashes.
At 103cm / 17.3kg, if you use the car infrequently, it is quite possible that she will get to age 4 before she reaches the limits of the seat you have in there currently. You could wait and see. I would also double check the orange label on the Pearl, depending on which exact one it is, some of them have an 18.5kg or possibly 19kg weight limit IIRC. This could buy you a couple more months, especially since children tend to grow in "spurts" and can hover at the same weight for ages at this age. Maxi Cosi are also built to a very high standard, so I would not worry at all about running right up to the limits, obviously don't exceed them significantly. It won't be the original Pearl as that seat was not rear facing. The 2Way Pearl was the first version which was.
In case you do want options for replacement seats:
There's only one impact shield seat on the market with the approval over 105cm and it's the Cybex Anoris T which is much more expensive than an ERF seat and only lasts until 115cm. All the R44 impact shield seats are discontinued, and were limited to 18kg anyway. Plus it's not a good idea to keep using an impact shield if the child is much too tall for it, it just puts a huge amount of strain on their abdomen.
The cheapest ERF seat is Britax Safeway M, when it goes on offer for £170, which it does fairly frequently. The idea about a spare set of tethers for your Max-Way might also work as a stopgap measure.
You also have Joie Every Stage R129 / Graco Slimfit R129 which might be suitable if she's physically getting snug in the Maxi Cosi and/or reaches the weight limit first - these are about £150, sometimes less, they are belt fitted and do rear facing up to 105cm/21kg, and then convert into a high back booster. The safety rating on them with ADAC is actually better than the Britax Evolvafix (which is one of the best rated FF harness/booster seats). Good option for a less-often used car. I would probably not exceed the 105cm height limit in the rear facing mode, because of the possibility of tipping the weight balance too much over the seatbelt fitting.
For forward facing seats, there are basically two kinds - you have the old "Group 1-2-3" type seats which have the 5 point harness in "Group 1" (up to 105cm under i-size) - the i-size versions of these seats are worth a look, especially Britax Evolvafix/Advansafix and Silver Cross Balance i-size (Essentials is also OK) because they have a 21/22kg weight limit and while you technically shouldn't exceed the 105cm height limit, as long as the child still physically fits, it's unlikely to cause any serious issues because of the way the seats are designed. The Britax one in particular has a block on how far you can extend the headrest anyway, so it can't be used too much over the height limit, but exceeding it by a centimetre or two is unlikely to cause any problems. The Britax seats also have a lap belt middle strap for the high back booster mode, which some people like. These seats are anywhere between £160-250 depending on sales, version etc.
Or you could look at an actual extended harness seat. It looks like Kiddies Kingdom have stock of the older Joie Bold R. There is also Cozy n Safe Hudson or Excalibur, those you can often find - the older, R44 version rather than R129/i-size, have a 25kg harness limit with no height limit. These are about £100-120.
There are two newer seats I know of which do extended harness under the new regulation - Cozy n Safe Excalibur i-size, which is harnessed up to 125cm but the weight limit might be about 21kg on this according to one random video I found about it, their manual is still not up on their website. Or Joie i-Irvana Max (must be "max" version) which is 125cm / 25kg on the harness and their manual is online which confirms. These seats are more expensive around £200. But the newer regulation is stricter, and they are easier to use.
The Joie i-Irvana Max would probably be what I'd pick if I needed an extended harness, FF seat.
If I did not need extended harness, I'd go for Britax Evolvafix (I have an Advansafix and am happy with it)
Joie/Graco R129 RF/booster seat would be my budget pick for RF.