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I am revisiting the Thereva inornata ♂ hypothesis for my specimen, based on three sources:

Mark van Veen – Therevidae of Northwest Europe
Mortelmans & De Bree (2022)
Haarto & Winqvist (2006)

For reference, the original discussion on insecte.org is here.

Using van Veen’s key, I arrive at T. inornata, mainly because my specimen seems to show:

basal posteroventral setae on hind femur 3;
tergites 4–5 mostly grey/pruinose;
a rather narrow dark anterior band on tergites 4–5;
two pairs of dorsocentral setae.

However, after reading Mortelmans & De Bree (2022), I understand that males of T. inornata, T. nobilitata and T. handlirschi are very close and should ideally be confirmed using terminalia and chaetotaxy.

As I understand the published distinctions:

T. inornata

hind femur with basal anteroventral row plus 1–4 posteroventral setae;
hind femur chaetotaxy usually described as nearly or usually uniserial;
tergites 4–5 mostly grey/pruinose with a narrow dark anterior band;
two pairs of dorsocentrals.

T. nobilitata

hind femur with only a basal anteroventral row, without a distinct posteroventral row;
anteroventral setae on hind femur more irregular, with displaced/additional setae basally;
chaetotaxy considered multiserial;
may sometimes also have two pairs of dorsocentrals, so that character alone is not enough.

T. handlirschi

hind femur with only a basal anteroventral row, regular, without displaced/additional basal setae;
chaetotaxy uniserial;
scutellum with dense yellow or almost entirely yellow pubescence;
also with two pairs of dorsocentrals.

So, the main point seems to be the hind femur chaetotaxy:

T. inornata: posteroventral setae present, chaetotaxy nearly/uniserial;
T. nobilitata: no distinct posteroventral row, chaetotaxy multiserial;
T. handlirschi: no posteroventral row, chaetotaxy uniserial, but with a distinctly yellow-haired scutellum.

Ecologically, the habitat of my specimen also seems worth mentioning: it was observed in a lowland Mediterranean area, in open agricultural plain near freshwater and brackish marshes. From van Veen and Haarto & Winqvist, T. inornata seems more associated with sandy habitats than with mountain habitats strictly speaking.

So at the moment T. inornata still looks like the best fit to me, but I would be very interested in your opinion, especially on the chaetotaxy of hind femur 3 and whether T. nobilitata / T. handlirschi can be excluded here.

Many thanks for the replies already provided, and also for the documents and references that have been shared so far.