I had similar woes with my Balance 30, so I can only imagine what it is like with the Balance 20. One thing that did help tremendously for me was to put a gigabit switch in front of the Balance. If you can keep LAN->LAN transfers outside of the router – it helps.
It sounds like you have this thing doing more than just DHCP. Inferring from your post, it looks like you are using the firewall to limit access to the internet for some devices, you are using the internal DNS server for custom DNS entries, and it is all on one untagged Lan. You never mentioned how many WAN links you are using - but, I would assume that it is more than one.
Some of those IoT devices (assumption) can be very chatty with multicast “discovery” traffic. Maintaining large multicast groups can also be taxing on routers.
Another option (one I used until I replaced my Balance 30) was to put a cheaper BestBuy router in between the Peplink and the LAN devices. Basically, you use the Balance to manage the WAN links and routing, and you let the “other” router manage the LAN devices. You have to deal with double NAT potentially, and another potential point of failure – but, it saves a few dollars. If you find a router with IP-Passthrough, you can use all the load balancing algorithms, if not – you are kind of stuck with destination based routing since the Balance will only ever see the IP of the intermediate router.
Free advice – skip on the TP-Link multi Wan router – it is a hunk of hot garbage. I had one for a year and was waiting for some firmware to make it useful enough to actually use. They still have not released firmware for it. None of the UPnP stuff works at all and it requires a reboot to clear any forwarders that are there. In fact, rebooting is the ONLY way to get that hunk of garbage to do anything. I still have it in a box – I would gladly give it to you – but, I am pretty sure you would give it back.
For what it is worth, The Balance One Core is a great unit. No issues from me for over 2 years and I have a similar number of devices and use a very similar feature set as you. Expensive, yes; but it does do the job very well and uses a UI/rationale that you are already familiar with. There is value in just getting it going and letting it do what it does.