Good to hear you are trying some things to find the breakup in the Chain of Hardware used and also interessting what you have found out. I will get into some points. But, and this is important, its just a guess because i have not seen your Hardware in RL.
Dish/LNB/Cables to your SAT IP Source the Digibit Device: Judging by what your saying/sayed it think there is no Problem at all, expect bad Weather that can come into Game and produce Continuity Errors like it does in my Setup when its strong Raining for example.
Telestar Digibit Twin SAT-IP Device: I also not really see the Problem on this Device, as long the Power Delivery to the Device it self is fine and the Device it self has no Overheating Problems.
Network-Cables/Switches: I also think there should be no Problem at all
TVHeadend Hardware (Raspi): Your Raspi should be able to handle the Input/Output of 2 Connections into each direction the same time even HD Channels since its not Transcoding anything. I Explain why i think so. Its having max 300Mbit NIC Speed and lets minus Overhead so we end up having 250Mbit for real use. One HD Channel will max out at 20Mbit, so by 2 Inputs from the SAT-IP it would be 40Mbit. Delivering these two 40Mbit HD Channel as Output to your Clients would add another 40 Mbit so we end up having ~ 80 Mbit wich the Raspi must handle and this is by far within the 250Mbit Range wich is safly available. CPU usage while doing this should not be intense and RAM usage if you dont use Timeshifting extensive should also be no Problem. But there is a IO/Thread-Handling going on while doing this and maybe this is where the Raspi hits some limits, even if it shows low CPU usage, but thats just a guess. My feeling is that its possible to let the Raspi do that but maybe its a little on the edge.
TVHeadend: Normaly there is not much to Setup when using it with a TV-Card (like PCI(e) or USB) attached to it and it runs nearly out of the box without Transcoding and passing trough Signals/Streams and it runs on potato PC Hardware. But SAT-IP can be a different Story. And this is for both different meaning for acting as Server or as a Client, and wich Medium its using as Source or where it delivers. I must clearly say that my live experince with SAT-IP is not enough to make 100% sure statements to be fair. If someone says SAT-IP runs 100% Errorfree in his environment does not mean it will do also in yours because the Setups may differ in/on Hardware.
Kodi or LibreElec: I also think there should be no Problem if using standard TVHeadend plugin up to date.
Following some Points of your Testing statements.
You sayed you configured a second RPi3 b+ and the Continuity Errors are gone now or it shows way less Errors. It must be sayed that TVHeadend is very agressive in finding/showing Errors compared to lets say a Tuner in a Consumer TV. I give you a example. Before some days a Specific Mux showed Problems in my TVHeadend and i got Artefacts on all the Channels on this Mux. So i switched the SAT-Cable from the TVHeadend TV-Card to my Sony Bravia and no Problems @ all even the Signals are shown way better (100% for both Signal and SNR) then shown in TVHeadend (~ 75% for Signal and SNR). Mysterous... So i get one Cabel hooked to the TVHeadend TV-Card and one to the Sony Bravia and watching the same Channel on both Devices with looking up on both Devices the Signal Strenght and Erros, and while TVHeadend shows Continuity Errors for a few Seconds and on Kodi in the exact Moment Artefacts, the TV shows a drop on 5% from 100% on the Signal for a few Seconds but no Errors and Artefacts. So this shows TVHeadend and Consumer Devices like for Example TV-Tuners handle or show Errors completly different. But, ithink if TVHeadend shows Errors, there are really Errors, even if they are not noticed in Artefacts or Frame Drops and Consumer Devices like TV`s or Recivers are just handle them different. So, yes if TVHeadend shows Errors they are real and for me, i like that to be truth.
About your DVB Viewer testing on two different PCs. First, using a Wired Connection if available is always the best Solution because a unstable Wireless Connection can cause Hickups if your Acesspoint is far away or behind many walls or there are too much other WLANs on the same Channel as yours disturbing each other. So, Wired Connection if possible, yes prefer it. And second, about your PCs Hardware. The Player must Decode the Stream that comes in MPEG2 (SD-TV), H.264 (HD-TV) or H.265 (HD-TV and UHD-TV) and most, even older dedicated GPUs or CPU with integrated GPU`s, can handle that fine up to H.264 without any exessive Upscaling or Postprocessing. Even your a little bit older Core i7 or Core i3 integrated GPU should handle that absolut fine without any Problems. If you want to use strong Upscaling and Postprocessing it maybe needs a little bit more Power then without it. Even my old HTPC CPU AMD A4-5000 with integrated Radeon HD 8330 iGPU (from 2013) did a very well Job up to H.264 with Upscaling to 1080p50 and Postprocessing, so you see it does not need very strong Hardware for Playback. Without upscaling and Postprocessing it is also possible to let the CPU (Softwaredecoding) instead the GPU/iGPU (Hardwaredecoding) do its Magic without any Problems, even on older CPUs.
If you are planning to switch from your Pi3 to more potential Hardware with Long-Term i would recommend you a little NUC with a Intel N100 or N150 CPU. They Cost around 100 Bucks (new, used maybe cheaper) and deliver way more Power in Terms of Performnce per Watt, many more and faster Ports for USB and NIC then a lets say Rpi5 with PSU and Case. these NUCs can easy be a great homenetworkserver for all your needed Services including TVHeadend, just setup Proxmox and install the services you want to serve your Home as VM or LXC and your upto go. My Geekom G3 Plus, acts as Pihole, NAS, SMB, PXE, mDLNA, Homeassistent, TVHeadend and Testing Server and is doing more then good for the next 10+ Years, it costs me ~ 130€ with 16GB RAM and 256GB NVME SSD, WIFI6 and 2,5Gbit NIC and It uses around ~ 8 Watts. Thats a no Brainer.
So i hope i could help you with my thoughts and if there are any open Questions just let me know.
