Whatever tool you use, you ideally want your lowest tier to consist of platforms that are already indexed - this is why blog comments and guestbooks are usually the go-to for T3. If you feel you have to send your lowest tier to an indexing service you've essentially wasted your time in building it. I think blog comments is the only thing I would use GSA for these days.
gsa ser can still find its use in tier 2 and 3 analysis, but for me, it's worth evaluating based on link quality and indexation, not only numbers. gsc should be used as well, because sometimes indexers' dashboards do not show all the picture.
Actually i would rather focus on the number of links that remain indexed after 30 to 60 days instead of the initial index rate because automated software allows for thousands to be produced.
Yes, that's my opinion about testing in a small batch. Another thing you should look into is whether the linking sites are indexed or not. If not, then you'll be just automatng the links without much effect.
LOL, you're already tackling the right fundamentals. Start off with search intent, internal linking, and then seeing if the pages are indexed in Google Search Console and getting impressions; backlinks can be added later.