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EU DTAG BW
I have to work around HT/Tcom network issues, HT is mostly pure DTAG (the Cogent is unusable and i know no Retn providers in EU) - https://bgp.tools/as/5391#connectivity
I look for either DTAG - AS3320 - direct/singlehomed VPS/dedicated or someone with DTAG upstream and fair routing (not cost based, or at least route DTAG "customers" like HT via DTAG). Right now i use a server behind Seabone in NL which is ok (peers off in AMS) but still has congestion issues at peak (might be the link ZAG-MIL-AMS). Stable latency and speed matter more than top speed and proximity.
A Retn singlehomed provider might be an option but getting burst 10G will be hard there for sure.
Try tracing 83.131.113.111, im not much interested in CIX/local servers as then i have to deal with upstream overselling in zagreb again anyway to go international (brr HE, ew). A solid DTAG standard (3 locations in EU+) PEERING might work well if not extremely oversubscribed.
In the past i had servers at Emsnet i think it was, in some small place in Germany (EMden, consequently), it was pure DTAG on DTAG AS reasonably priced but probably long gone.
Telekom cloud might be ok (same as HTs clone local hosted in ZG) but is expensive af.

Comments
GLWR
@ComputeBox has DTAG 3320 as their upstream in their Hamburg location. i ran a traceroute with one of their vps i use and it was around 30ms and routed through DTAG
Seabone looks like this from AMS, it and GTT are the only widely used T1s with sufficient DTAG EU capacity at peak (yes you can get to 3320 native from Cogent/174, but its not a nice trip behind the hops, ask your latency) - i found no usable singlehomed Level3, TATA, Orange (Free.fr is sadly depeered and OVH extremely oversubscribed) or Arelion (found some Nordic ISPs with it but not much) ISPs in EU and testing everyone multihomed is not feasible.
AMS network
Seabone upstream, not peering
7.|-- 213.144.161.220 0.0% 8 1.6 1.5 1.3 1.6 0.1
AMS
8.|-- 195.22.213.204 0.0% 8 1.8 1.8 1.6 1.9 0.1
DTAG ZG core, by latency via ZRH/MIL not FRA/VIE
9.|-- 195.22.213.34 0.0% 8 38.9 28.7 14.0 38.9 8.7
HT ZG core/BGP
10.|-- f-ed12-i.F.DE.NET.DTAG.DE 0.0% 8 43.0 32.5 18.2 43.0 8.5
Rijeka, HT near train station
11.|-- 80.156.162.77 0.0% 8 51.2 43.6 31.6 51.2 7.5
Fiber split unit office in Kastav
12.|-- htr11-gtr11-2.ip.t-com.hr 0.0% 8 47.3 45.3 33.2 53.5 7.7
BGP
avoro has dtag, worth a try?
Yes, it seems CSL (their upstream or parent) pays for normal DTAG transit, the route looks ok (Zagreb - Budapest - CZ and FRA or HU and MUC - Hamburg). Sadly inbound is cost based (Twelve99) so they are likely not usable. Sent message for test file though.
CSL Hamburg
1.|-- 195.60.92.193 0.0% 10 0.2 0.4 0.2 0.5 0.1
DTAG BGP
2.|-- 87.128.232.17 0.0% 10 3.9 3.9 3.7 4.0 0.1
FRA
3.|-- f-ed12-i.F.DE.NET.DTAG.DE 0.0% 10 10.2 10.2 10.1 10.5 0.1