I think you need to remove ether2 and ether3 from bridge-local also make sure they don't have master port set. After bonding you'll have a new interface which you can add back to your bridge. Warning: I could be wrong, I haven't tried link bonding before.

https://mikrotik.com/product/RB750Gr3#fndtn-testresults

This one should be ok on fastpath mode enable

Or wait for RB4011? Any news when it will be available in Malaysia?

A few quick tests on "my" Turbo500 and RB750Gr3.

Fasttrack On: DL: 530Mbps UL: 110Mbps
Fasttrack Off: DL: 270Mbps UL: 110Mbps

5 simple queues (1 900M queue for the test PC), 19 firewall rules (including the 2 for Fasttrack), no mangles, vlan50 for LAN traffic.

Queue for test PC removed.
Fasttrack Off: DL: 350Mbps UL: 110Mbps

Will test again when I get my 800Mbps Turbo.

depends on what model you got.

I have the old RB750G (680MHz CPU) <-- this was a rather popular model in old times, I think a lot people has this.

30mbps legacy unifi package = 30% CPU
100mbps unifi package = 65% CPU

both are fast track off @ gigabit LAN @ only 1 desktop PC. 

I think this router is capable up to 200mbps only despite it has gigabit WAN port.

Export the configuration to file then import into new router.

You will need to enable fasttrack in order to achive higher speed on your RB750Gr3. If you need to use your queues and other fancy filters, then you can't use it. If you need your queues & filters, then you didn't make sufficient research before choosing this router. Although I do know hap ac2 without fasttrack will do fine with 500mbps symmetric, I'm not sure with 800/200 unifi. I've seen test result from a ukraine site that shows single connection on hap ac2 with pppoe will only yield 500mbps and will require additional connections to reach the max throughput. This results is without the use of queue though.

My advise if you don't know what you're doing, factory reset & use the default firewall rules as the starting point and recreate the other rules that u need. I doubt anyone willing to help you fixing the messed up firewall rules & configurations that were made following outdated guides.

How old the cpu in the computer you using? Older gen cpus are affected badly by the spectre cpu microcode update. I've seen x79 system having problem getting even 400mbps throughput with antivirus enabled. It is even worse if the gbe adapter is on USB. Check the cpu load of the router and your computer when you fully load the internet connection. If 1 of your computer cpu core fully loaded then it is likely due to the spectre bugfix. If it is the router cpu being heavy loaded, then it should be your configuration issue causing the FastTrack rules not working or it could be some functionality that has been enabled and interfere with it. Check the FastTrack counters. And also don't use ipv6 if you need performance improvement of fastpath/FastTrack, which I'm not sure if it still the case as I don't keep track of their new development.

PPPOE and VLAN will slow down your speeds, so the wirespeed claim of the HEX doesnt work because while NAT is accelerated, PPPOE isnt so the 480Mb/s you get is very similar to the speed rating given for accelerated VPN by the router.
i would suggest netgear's smart switch. Whenever someone asks for PoE the 1 rule i always mention is that it needs to be a managed device, because a dumb switch wont tell you whats going on and where your problems are (like what volts, amps, watts, or if its working). The ability to report a status is something i consider very important for PoE, otherwise you might as well just get an injector instead.