I recommend that while you are learning you get a 2020 Almanac. The practical duration of an NA is one year as described in the “Explanation” section of the almanac (pg 261). You can use the 2020 almanac for 2021, with corrections, but it cannot then also be used for the Moon or planets. HO 249, Vol 1 is good for +/- 4 years on each side of the Epoch. Perhaps that is what you are thinking of. Purchase the “Commercial Nautical Almanac” (perhaps from Celestaire, for less than $30.00, which is less than half of what the Gov issue is).
In regards to the bubble sextant attachment from Celestaire. When you replace the sextant scope with the bubble sight attachment you will notice that the attachment more resembles a tube rather than a scope. The narrow, optical entry point for the attachment, being smaller than the scope’s optical entry point does not physically block the reflected light from the horizon mirror from your eye as does the scope. Also the other end of the attachment (the ‘eyepiece’) also does not have an eye shield as does the scope. If your eye is not fully on the scope( properly lined up with the eyepiece), it may be possible for you to see reflected sunlight directly from the horizon mirror as you move the sextant about. I have noticed this, but it has not been a problem as it occurs rarely. Light is supposed to leak into the bubble sight attachment to help illuminate the bubble and depending upon sextant position it will sometimes appear brighter than at other times. This is by design and is not a problem. Hope this helps.
The technique I often use with a watch, when timing my sights, is to start counting immediately after saying “Mark”. In other words, “Mark one-thousand-one, one-thousand-two…etc until I can look at my watch, stopwatch, time display, etc. This works quite well for me. When using my smart phone with an app I find it easiest to have the phone standing-by on a flat surface, within easy reach, with the app active, so that I am not holding the phone, and that I can get to it quickly.
In regards to electronic aids of any kind, as an aid to classical, traditional Celestial Navigation. Ultimately you must be very proficient in determining your position without any additional electronic assistance. Learning independently, and while learning, I would recommend getting the latest paper almanac and reduction tables to learn how to use them and to appreciate all that they offer. Study them as stand-alone separate tools. Non-electronic tools such as the Starfinder, star charts, etc, are invaluable as well. Use these tools as you primary sight package components to work your sights, and then, as required, augmenting the accuracy of your results by using a calculator, apps such as “GPS Anti Spoof”, etc to hone your skills to the point that you have the confidence to take quality sights and to then determine your position accurately. After this you can start playing with old Almanacs and tables to your hearts desire with the developed ability to decide for yourself whether the extra work is worth it to you.
By the way, there is always something to learn or to improve upon when doing CelNav….the sky is the limit (poor pun intended) Like the rest of us, you will always be a student. Good luck!