Most versatile 123 seat (Reluctant FF)?

I'd look at Britax Advansafix, as that is the best rated (2.3 for safety) 123 seat with 5 point harness overall. You will need isofix and top tether to fit it, but it fits in any car with those features. It also makes a very good high backed booster.

Any car produced after 2013 will have both isofix and top tether, so it's not a problem if you're looking at second hand cars. Some older models do as well, it's completely impossible in cars older than 1997, then 1997-2006 it was rare, 2006-2013 more common, but not every car. You can look for the top tether points in the boot before you buy.

Recaro Tian Elite and GB Everna Fix are also some that got a decent score, but I think they're just not quite as nice as the Britax.

The Evolva is the best rated seatbelt fit one (if you exclude impact shield options, which TBH I would for a 3.5 year old) but it's quite a step down from the isofix + top tether options - 2.7 for safety. Although there is an isofix version of the Evolva, it is not truly isofix and just uses the hooks for stability. That version has slightly better side impact protection.

The scores I'm talking about are from ADAC who inform Which? - the possible score goes from 0.5 - 5.5 and lower numbers are better. Forward facing seats with 5 point harness tend to score between 2-4. (Rear facing seats typically get scores between 1-2). Ideally I would stick to seats with scores between 2-3, again, lower number is better. As you say she is small and light, I would not bother with the 25kg harness seats, as these tend to score worse in FF, so are only worth it if you need the extra weight or height capacity.

If money is no object, you could look at the new Cybex seat with airbags, the Cybex Anoris T which is eye wateringly expensive but can be used a bit longer than most Group 1 seats (115cm/21kg) - but this does have the impact shield which can be a bit controversial. While I wouldn't want to use a pure impact shield seat for a child of this age unless a last resort, the airbag one might be worth considering as it does look like it supports the neck well in the crash test video. It doesn't transform into a booster seat, so won't last as long as a 123 seat, and although the crash test rating is impressive, bear in mind that it uses an impact shield as a base function so they tend to score better in the first place - it's not directly comparable to a 5 point harness because those tests don't measure the abdominal forces placed by the impact shield nor the higher chance of ejection.

In general though I would be (am! My 4yo is FF too and has been for a while) happy with the safety of a 5 point harness in a decent branded seat FF a 3.5 year old - don't feel guilty if you can't/don't want to spend the money on the fancy airbag seat!