Changing to a larger vertical race fin is the least expensive way to get earlier planing.
Yes, an upgrade of as little as 4 cm in span can make alot of difference.
On the range suggested, I'd make the best early planing range in the 56-62 cm range.
If you need to buy a fin to get into this range, I would look for a 58 or 60 first and
56 or 62 only if you can afford them. 4-6 cm is a good "quiver spread" for vertical race fins.
In the USA, I like both Tangent Reapers or True Ames Shallow Water Weeds on the wider boards and True Ames Blade Weeds on my smaller boards.
For the Rio M, 17" (43 cm) or 18.5 (47 cm) will work pretty good.
I'd check out the Tangent Dynamics Reaper 42 and 46 to get into that size range. If you go with True Ames, the 39 cm (15.3") or the 44 cm (17.5") would be my choices for your board.
If you go with the Blade Weed, I'd suggest the 17" (43 cm).
The 18.5 would work, but might be a little "draggy" unless you are using sails in the 9.0m2 + range.
Hope this helps,
P.S. Remember, weed fins have alot more area than vertical race fins, so you can run smaller weed fins .