Which macro equipment option for "bugs" and flowers.

I would consider close-up lenses on your 18-250. If you get an RX10IV you could use the same close-up lenses on that too.

There are two sorts of close-up lenses. You mentioned a close-up filter kit. these have the advantage of being cheap and flexible in terms of the range of magnifications you can achieve with them. However, each of the three or four filters in the set is constructed from a single piece of glass and will tend to suffer from colour fringing, softness, and distortion, especially away from the centre of the image. Sometimes you may not notice, and sometimes it may be so serious that you wouldn't want to use the image.

The second sort of close-up lens is an achromatic close-up lens ("achromat"). These are generally sold singly and typically cost significantly more than a close-up filter set, each one costing perhaps three or four times as much as a filter set. However, achromats can produce very good image quality.

I used achromats for over 10 years for insects, spiders etc, and for almost as long for flowers. I used them on a 55-250 lens on an APS-C Canon 70D (similar to your A77 with 18-250) and used them even more, mainly for insects, spiders etc on a Panasonic FZ200 and FZ330 (both being 25-600mm like the RX10IV, but with a smaller 1/2.3" sensor compared to the RX10IV's 1" sensor).

Another alternative might be a legacy macro lens used with a converter on your A77. Can the A77 use legacy lenses? I don't know. If you can this may be cheaper than using achromats, of which you would need at least two, a low power one for flowers and a higher power one for insects, spiders etc. With a 1:1 macro lens you would get from infinity focus down to 1:1 without adding/removing anything such as a close-up lens or extension tube, and you would presumably be able to use your extension tubes to get more magnification beyond 1:1.

I am very curious as to what this 10X macro lens is for $80. I would not think that a 10X lens would be suitable at this stage. 10X is a lot of magnification, more than is sensible for a beginner to be using.

If you are going to photograph insects, spiders etc you may find you need to use flash, in which case you will then need to find out about, and experiment with, diffusion for the flash. The higher the magnification you use, the more likely you are to need flash.

It may turn out that the precise equipment you use is the least important factor in making good looking images of flowers and insects, spiders etc. More important may be fieldcraft (knowing where to find subjects and knowing enough about their behaviour to be able to photograph them without frightening them away), illumination (whether natural light, flash, continuous lighting or some combination), composition and post processing.