I edited your post for clarity, I hope you don't mind. Self servicing of your regulator is a very practical solution for someone with some mechanical skills and inclination. It's not difficult, but you need to know how to be careful, you need to have some basic curiosity and ability to understand how things work, and you'll need someone to teach you a few things. You don't need a certificate to do well, although you might find buying parts for some brands easier if you take their specific course on service. The rules about this probably vary quite a bit from country to country.
With regards to buying an additional regulator and converting the one you have to SM, as long as your current regulator is DIN and has an end port and/or turret, that's a fine idea. I don't know what you were referring to when you said "the screw and constant changing". For me, occasionally I do a trip where most of the diving is SM doubles, and a few OW dives with a single tank. To facilitate that, I have to bring a longer HP hose (24") and a longer LP inflator hose (14") and I switch the hoses around. Is that what you were talking about? If so, worry not. But it does sound like you'd have a ways to go before you understand enough about regulators to service them yourself.
I wouldn't get too worked up about this; again, any of the regs you mentioned will work absolutely fine, just make sure you have the turret and end port. Most people who do well in SM have had some instruction in specific SM set ups and adjustment, not to mention all the new skills that diving SM requires to do really well with it. I have seen a lot of pretty bad SM divers, and you don't want to be one of those. So find an excellent SM teacher that can help you, and he or she will very likely have good advice about the regulators.
No problem! Much appreciate to be honest! And also thank you for making it very clear what is the main issue. I want to learn more about it but don't have many sources to compare.
The current one is the Apeks DST with XTX50/XTX40 and without the 5th port. It is DIN (My instructor in Thailand would slap me if I bought yoke, lol). It is as you expected, "the screw and constant changing" is the hose configuration and that stuff. Just worry that I can over-tighten or like cause unnecessary wear on them, which can cause leakage. But now that you mention it, I am not sure how often I will dive sidemount.
I don't mind learning to self-service my regulator. It looks very interesting, but from what I see, it seems very common for people to self-service their own regulators than send them off. A little surprised but glad that it is a norm here.
Speaking about the setup, I have some questions outside the regulator discussion if that is fine with you.
1. When you diving sidemount with a regulator that is used for single, what do you do with the leftover octo? Do you keep it or remove it?
2. I understand that sidemount BCD is good as it wraps around the body and optimizes for sidemount. But is it possible to start it with the backplate that had the necessary attachment? (Buttplate, D-ring, Shoulder Bungee, etc...)
3. In Japan, 10L aluminium tank is pretty popular, does this mean that if I go to Thailand with 12L aluminium, I need to readjust the trim on sidemount?
4. I saw an instructor dives in sidemout BCD, but with a single tank during for recreational dive. I just wonder how will he share air if it has only a single second stage on it.
5. Is it common/hard to dive sidemount but with only single tank?
Thank you for all of the comment and insight on this stuff!