Depends on 'You' and how you want to interpret 'Relaxed position' - and your general Body dimensions...
Here's a link to a comparo of a 2020 V3 - 54 & 2023 Basso Astra in 56 & 58 - which seem to match your numbers...
... on the 56 a .5cm shorter reach for me is not a huge consideration - could be compensated by a longer stem, unless the integrated cockpit/stem/headset thing is complicated ... the very slightly steeper seattube angle combined with shorter stock stem length might make it 1 cm 'shorter' reach.... a big deal ?
AND the reach for the Astra 56 & 58 is only 2+mm different - then the stem length difference makes up whatever the actual reach might be - frame difference seems negligible...
The 58 have a 4.7 higher stack would be a sizeable increase - depending on what you want...
Since neither lists a standover height & Both the V3 and the Astra have a 'Seattube Length' listed, but not as 'Effective Seattube Length - I have to assume the standover on a 58 is quite a bit higher than the V3 and Astra 56....
so what your cycling inseam is, might be a consideration... or not...
...maybe the shop has a 58 in stock ? or can;t get a 56 ? or this is how they interpret your desire ?
... I could say what I prefer... but that shouldn't impact on your decision...
.... if you can replace the stem on your V3 - buy a cheap +17 deg. stem, and you'll get a feel for the difference +40 mm stack will be....
//yojimg.net/bike/web_tools/stem.php (I put in numbers: 73.5 angle, 120 length & --7 deg for V3 & 73.5, 110 & +17 for the Basso...

Ride On
Yuri