I mean as you know leather engraves dark without much power at all. Honestly, it’s hard to say why it’s apparently overburning and losing details at lower powers, but I suspect it’s your lpi and speed combination.
This is probably counterintuitive, but I might try slowing down the beam. You get better resolution of small details if you bring the speed down a bit. It has to to do with laser on-off latency, I believe. I can’t give you specific speeds, but I might bring it down to 300 or lower with your very low power levels. I saw similar speed-based issues when I was engraving my mink skull.
As for lpi, you’re delivering a lot of laser at 450 lpi, hence I think you are seeing a lot of burn at such low powers. 450 lpi is a bit overkill for almost anything. Judging by the scale of what you’re making, and the leather media… 270 lpi will probably give excellent results in a fraction of the time, and allow you to get finer power levels dialed in.
As for getting masking in person, you got any vinyl signage shops nearby? They sell it as “transfer tape” or “transfer paper”. You can get some transfer tapes at like michael’s or other art stores, but do not buy them if they’re plastic. You need paper masking, the plastic might be laser safe, but it probably won’t look great when it’s done.
The price in person will likely hurt compared to the options I presented, but desperate times…
(moved to BTM)