Hi there,
I am very pleased with this upgrade kit. And grateful for it. My prayers answered. However i did notice a few things inspecting the PCB. And just wanted to let you know. In case you decide to make revisions or updates for future runs.

Most of these are minor small points. Nothing really too important. So please don't take it as any criticism - you have done an excellent work. And with all the most important aspect really well done.

You can see here is a numbered list, for each point.

1. Is there any use here for RTC battery? Is it meant for having a clock or something? Are settings not need RTC, but instead they stored and remembered in the flash - yes? ...Because if there is no need for it at all, then I can instead cut the trace. And re-use those same terminal points for jumping to some other pins. With a mod wire. Please confirm if it does not matter.
2. When soldering this GND pad, it was more difficult. Because of course the ground plane. However maybe if you make some cut out (gap) around the GND pad... then that will thermally isolate enough for an easier soldering experience. This is good for example here the LCD screen does not draw much current, so the ground connection is not so critical. It not matter (for me). But maybe some new user or person with a bad soldering iron. Then you can help them not to struggle so much.
3. Here I can see you break out all the unused GPIOs - great! very good. However in the middle of this header you put the only breakout for the SWD debugging / programmer IO (SWD_CLK and SWDIO). This is not so good to take here (for SWD debug). Because there is no GND or +3v3 nearby. And no NRST pin. So what you want to do is keep it there. But also add dedicated separate SWD debug port. And include the other useful pins next to them. Perhaps something like this: [ 0v, SWD_CLK, SWDIO, 3v3, NRST ]. Or however you want to include them.
4. This is NRST, ok no problem. I checked with DMM
5. Buzzer polarity: I was not so sure which way around to install it. However reading online it seems to depend on the device whether polarity matter (or not). But I did check and got higher voltage from one side, which I guess means it has more space to move on the top side... it does not matter for me. I don't mind at all. Just thought to mention it. As perhaps it can make the buzzer sound louder (or more quiet). By reversing the polarity. Which is useful, to the preference. Of course there are also other easy ways to make it quiet. So no problem
6. What are these for?
7. Here is BOOT0. It is also useful for programming. However I could not find the trace, or know if the leg is broken out to any pin. I think this is good idea to make a pad for. In case we need to do programming for the open firmware. Typically you just make a 3 pin jumper, [ 3v3, BOOT0, GND ]. To pull it high or low (while flashing). Otherwise it floats internally

So that is all I can see here for suggestions to improve the PCB.

Programming / Debugging

• Measuring the current for passively powering the 3v3 rail from the programmer. It draws only 43mA, which is great. Enough for the programmer to power the device during flashing etc. Without browning out. OK then!
• While for passive live debugging (independent power). Then we do not connect the 3v3 pin. Only the GND pin, to maintain a ground reference.
• For next steps, I have now made a programming cable (for SWD). To try connecting and flashing etc.

Actually you did a really great job for the firmware. I do not plan to change very much. Except maybe if I require more than 3 presets. Then perhaps the buttons can be changed to scroll through numbered list of presets. Up, down, and select. Instead of directly switching to the preset.

However I will not add this until I really know that I need to keep more than 3 presets. So perhaps it won't be needed, then I will not add this.

And of course, as you also mention in your video: it is great to use the GPIO header, for connecting another slave device to control. For example fume extractor on/off. Or for attaching another input device, such as a foot switch pedal(s). So that hands are free. Thank you so much my friend!