Have a look at digikey for Bridgelux COB's but make sure it's gen.7. They ship to EU and you don't have to pay custom fees only VAT. All the prices are net prices and VAT is different and depending on the country.
I would use 3 or 4 strings of 3 COB's, 9 or 12pcs COB's in 3 or 4s3p. Each COB would run with ~24w/700mA or 17,5w/525mA and you can mount them to the exsisting fixture. I'm using ⅔ CRI80 F-strips and ⅓ CRI93 COB's. F-strips are running with ~525mA and COB's are at ~500mA and I've used the fixture at 12" with no issues like bleaching.
The good thing with low driven COB's is you need no expensive heatsinks. A modded aluminum C-channel with 30x 40x 30mm, 4ft. long is enough for 70-80w low driven COB's. See the 400w light in my signature, it has two modded C-channels each with 4 COB's at 500mA(70w) and highest temp reading was 116°F/47°C with 84°F/30°C ambient temps.
You only need to glue a small c-channel into a bigger c-channel to increase the surface area and convert it into a long passive heatsink.
For UVB I would recommend to use good reptile bulbs. 24w each 0,5m² is enough because the daily dose is determined by hours of usage! Agromax has indeed the PureUV which is 3 times more powerful but they do not ship in the EU and there is no official EU distributor till now.
But good reptile bulbs like the Arcadia D3desert(12% UVB/30% UVA, 7000°k/CRI90) or D3dragon(14%/30%, 7000°k/CRI90) used for 3-6h the day gives you more than enough of these high energy wavelengths.
vat calculator
Lets say you use the 24w D3dessert 2ft T5HO bulb at 16-18"/40-45cm above the tops. You can expect 80-100μW/cm² UVB with alu reflector. If you use it for 6h the day(3h at the beginning and 3h before lights off) that's already a dose of 17,28 to 25,92kJ/m². Reflectors are a must have with those 360° bulbs and target is to distribute the light as even as possible without loosing too much on the walls.
