First things first, what do you mean with "The switch isn't going to negotiate 2.5gb"?
Of course it won't do that on the 1Gb ports but you have like 4 SFP+ ports that have 10Gbit bandwith each, what about those?
The SFP+ slots per-se don't force you to run at 10Gbit, the transceiver module you install into them has 10Gbit connection to the switch but then it can negotiate whatever speed it wants on the wire.
Did you try already to buy a SFP+ module for your switch like for example this (unaffiliated, just random one that actually lists some specs) https://www.amazon.com/QSFPTEK-Transceiver-10GBASE-T-SFP-10G-T-S-UF-RJ45-10G/dp/B07QXNQTXG/ and confirmed that you can't just plug a cable that goes to one of your 2.5Gbit ports?
Here also another one of Mikrotik (a more well-known brand) from another website https://www.roc-noc.com/mikrotik/routerboard/SplusRJ10.html same story, can autonegotiate whatever it feels like on the wire.
So as long as you buy a module that says it can autoneg down to 2.5Gbit you should be fine.
Getting a module and connect a wire is the "recommended" and "best performance" and "smooth-brain-proof" choice. It should work as long as your switch does not require special transceiver modules due to stupid vendor lock limitations like for example Cisco.
I mean yeah, it will cost you half of what you paid for the Odroid, but hey, it could have been worse, you could have been stuck with bonding.
usual bonding disclaimer:
Bonding, especially done in software like this is not what most people think it is, you join the total bandwith of the network but the max bandwith per connection is still the same (you can make more connections). So for example if you bond two 1Gb connections, two applications can run at the same time using 1Gbit without hurting each other, but each can only get as much as 1Gb due to limitations in TCP protocol.
This is what you also experienced with your iperf3 test. Max speed per connection is still 1Gbit, because surprise surprise that's the size of the ports you are bonding.
Some businness switches may or may not break spec and actually pull off true bonding like people think, but that's not standard and won't work unless both switches are of the same brand, and very not the case here so let's not get into that.
For the hardware support
https://bugzilla.kernel.org/show_bug.cgi?id=208361
The support for that driver is merged in upstream Linux kernel version 5.9, OpenWrt snapshot is still on 5.4, and testing kernel 5.10. Good luck waiting on that.
Since your device's hardware specs are on better than most entry-level firewall appliance anyway, OpenWrt is not your only option.
There are pfSense or OPNSense, and you can run either of those OSes instead of OpenWrt. Both at latest versions should support also the rtl8125 NICs and still offer a web interface, documentation and a full "serious firewall" experience.
Both can do link aggregation so you can do what you wanted to do.
https://docs.netgate.com/pfsense/en/latest/interfaces/lagg.html
https://docs.opnsense.org/manual/other-interfaces.html
But what I said above about bonding limitations still applies even to them. SO yeah, you won't go faster than that.
how I can get an ipk for luci-proto-bonding on 19.07.7? (besides building it myself lol)
Luci is a bunch of scripts and text files, you can try just installing whatever is in snapshot and see if it does work.
If it does not work, then it's not a matter of building (there is nothing to build), but of backporting what works in snapshot to what Luci was using more than a year ago.