Hi,
I came up with a Colemak-like layout, but optimized for English and additional languages like German and Dutch.
Tarmak-like, switch a few keys at a time
I first tried to just switch a few chars from qwerty to my layout, so similar to the Tarmak approach. Because I developed my own layout I was very aware where each key should be in the final position and why it should be there. So when I tried to keep my qwerty skills (~ 400 chars/minute = 80 wpm) I soon found out that this was hard, because sometimes I was typing the qwerty-key, but sometimes I already typed the key which was not changed yet, but I knew would move to another position. That was very confusing. I then tried another approach which worked much better form me:
Practice new layout -- keep using qwerty daily
I started to learn beginning of September and practiced on most days for about 15 minutes. A few days I missed, but in the weekend I often practiced about 30 to 50 minutes. So on average I was practicing 15 minutes daily. For that I almost exclusively used keybr.com, which I found to be an excellent way to learn the new layout. I mainly practiced on the German layout, but also switched to other languages (English, Spanish, French, Italian....). Also including the other languages helped when I was pretty confident with the current active characters, but to bring in some variation and to consolidate what I had already learned. Because keybr just generates gibberish words it does not matter that much which language one practices in and one can use the variation from other languages, which I highly can recommend.
The very first two days, when I started, were a bit hard, but then I made good progress and it was also not too hard to keep using qwerty for my daily tasks. I constantly made progress and improved accuracy and speed till I had learned all characters and reached a speed of 40 to 50 wpm with a relatively low error rate (~ 97 %).
The last days, when I almost had learned all characters, it began to get a bit more irritating to type in qwerty and I began to slow down in qwerty or make more mistakes (where normally I type without too many). Not "thinking" in qwerty helped. So I could type better, when I typed fast in qwerty, where I am very fluent in, then when I had to type slower, because I was typing an unusual word or phrase (like a new password).
Full switch
So when I had all characters under my belt and approached 40 wpm I decided to switch fully and not use qwerty any longer. A speed of 40 to 50 wpm is not that fast, but quick enough not to slow down daily work too much. So that worked quite well, but was still using much of my concentration when typing.
When typing on keybr or monkeytype I was much quicker than when I was formulating my own thoughts in an e-mail or the like. Then when I had something urgent I tried to switch back to qwerty, but at that point this was not working any longer. I guess one could try (after a while of using the new layout only) try to switch back to also use qwerty. I for myself have decided not to do that. I think it is not worthwhile for someone like me, who almost never has to use other people's computer and if that happens I only needs to type in a password, url or a few commands. Then I can hunt and peck, or type slowly blind in qwerty. My guess is that few people will be able to keep two layouts at a higher speed (above 60 or 70 wpm). It is possible I know. But why should I care for my situation!? Just makes it more likely not to make progress that fast or make errors more easily. Now with qwerty (I have many years typed blind) this process is totally automatic and I do not need to think at all. That's what I want with a new layout and am still not sure, if it is worth the efforts to learn a new non-qwerty one. But I'll try it. Partly to see how easy or hard it is to learn that and partly because I think the learning experience will help me in other areas like practicing an instrument as well. And for sure I also hope to be able to type more relaxed, which I think is true, because the hands will not need to jump around wildly that much with a better optimized layout than qwerty.
Decision to ditch "Colemak-like" and go full optimized
But what I now think is that it is not worth to switch to Colemak or a similiar layout like my own similar creation, because it still has too many drawbacks IMO. I think if one invests the time to learn a new layout it makes sense to go the full way and learn a layout which is more optimized than Colemak and possibly also make some adjustments to personal needs.
[EDIT]
I do not say that other alternative layouts are always better than Colemak or for everyone. That was just my personal experience that I felt the right hand was typing too long sequences. That felt "restless" and not that comfortable to me. When you look at the finger movements (see attached PDF in the thread linked below) one gets a pretty good idea how a layout will feel. At least now after I have experience with qwerty and "Colemak-like" and then compare that to words typed with AdNW or similar layouts I think most people will feel similar, but that is just my (now educated) guess. :-)
One possible argument I forgot to mention is that I am not sure if is better to switch only half of the keys (Colemak) or more or less all keys. To learn the first is surely quicker. But I found I made many more mistakes for the letters which just changed key, but stayed on the same finger. For example N (which I had on the qwerty-j Position), but also e (on the k-position) made a lot less problems like the d, which was on the qwerty-e position. So long-term it might be that a completely different layout might be better, especially when one tries to stay more or less fluent with qwerty.
I hope my experience and thoughts are helpful for someone.