When you use the word "fill", I just imagine a topsoil ripped off of a development and then re-sold. Probably a candidate for compaction in a bed. You are probably way better placed than me to comment, but supplied soils to raised beds tend to be higher in organics and structure is lost over time, even the sunken logs would be at risk.

I've posted about structural soil before, the gravel one if you remember. There are many types, here is one called Amsterdam Tree Sand - https://www.cpa-horticulture.co.uk/urban-tree-soil

Here is an search engine AI having a crack at it. The AI is literally defining it in the urban sense, involving pedestrian and cars, whereas I'm describing for containers:

Structural soil is a type of soil used for planting trees in urban environments. It is designed to provide a load-bearing substrate that can support pedestrian and vehicle traffic while also allowing for root growth[4][5]. Structural soil is composed of a mixture of gap-graded gravels, mostly made of crushed stone, and soil with mineral and organic content[5]. The gravel provides load-bearing capacity, while the soil provides nutrients and water for the tree roots to grow[2].

Structural soil is typically used in areas where trees are planted near sidewalks or roads, where the soil is often compacted and unable to support healthy root growth[4]. The soil is compacted to pavement design and installation requirements while still allowing for root growth[5].

CU-Structural Soil® is a specific type of structural soil that was developed by Cornell University’s Urban Horticulture Institute in the 1990s[1][5]. It is composed primarily of specifically sized gravel blended with a loamy soil that allows it to be compacted and placed beneath sidewalks, streets, and other pavement[4]. Trees growing in CU-Structural Soil™ in areas that normally use irrigation to grow trees should also provide low volume drip irrigation in CU-Structural Soil™ [3].

Using structural soil to support trees helps them develop healthy roots and grow to their full potential[6]. It also helps prevent tree roots from cracking or damaging concrete or asphalt[4]. Structural soil was once the standard for planting trees in urban environments due to its ability to sustain healthy tree growth in challenging conditions[2].

Citations: [1] http://www.hort.cornell.edu/uhi/outreach/pdfs/CU-Structural%20Soil%20-%20A%20Comprehensive%20Guide.pdf [2] https://citygreen.com/what-is-structural-soil/ [3] http://www.hort.cornell.edu/uhi/outreach/pdfs/custructuralsoilwebpdf.pdf [4] https://riversandinc.com/soil/structural-soil/ [5] https://en.wikipedia.org/wiki/Structural_Soil [6] https://www.minickmaterials.com/blog/using-structural-soil-to-improve-urban-environments